Multi-level discriminator handling improvements (#9119)

This pull request refines the logic for handling multi-level
discriminators in model constructor initialization within
`ModelProvider.cs`. The changes ensure more accurate passing of
discriminator parameters in complex inheritance scenarios, improving
both correctness and maintainability.

### Multi-level discriminator handling improvements

* Updated the logic to check whether the current model or base model
uses a multi-level discriminator, ensuring the correct constructor is
called and the appropriate discriminator value is passed in multi-level
inheritance scenarios.
* Enhanced the handling of discriminator parameters: now, when
initializing constructors, the code attempts to pass through the
discriminator parameter from the base model if available, otherwise
falls back to using the current model's discriminator value.
* Improved the logic in `GetExpressionForCtor` to correctly decide when
to pass through the discriminator parameter or use the model's own
discriminator value, based on whether the model is part of a multi-level
discriminator hierarchy.

### Constructor initialization logic

* Ensured that base constructor calls are properly made even when there
are no base parameters, maintaining correct inheritance behavior.

Author: radhgupta

docs/samples/client/csharp/SampleService/SampleClient/src/Generated/AnimalOperations.RestClient.cs

@@ -0,0 +1,45 @@
+// <auto-generated/>
+
+#nullable disable
+
+using System.ClientModel;
+using System.ClientModel.Primitives;
+
+namespace SampleTypeSpec
+{
+    /// <summary></summary>
+    public partial class AnimalOperations
+    {
+        private static PipelineMessageClassifier _pipelineMessageClassifier200;
+
+        private static PipelineMessageClassifier PipelineMessageClassifier200 => _pipelineMessageClassifier200 = PipelineMessageClassifier.Create(stackalloc ushort[] { 200 });
+
+        internal PipelineMessage CreateUpdatePetAsAnimalRequest(BinaryContent content, RequestOptions options)
+        {
+            ClientUriBuilder uri = new ClientUriBuilder();
+            uri.Reset(_endpoint);
+            uri.AppendPath("/animals/pet/as-animal", false);
+            PipelineMessage message = Pipeline.CreateMessage(uri.ToUri(), "PUT", PipelineMessageClassifier200);
+            PipelineRequest request = message.Request;
+            request.Headers.Set("Content-Type", "application/json");
+            request.Headers.Set("Accept", "application/json");
+            request.Content = content;
+            message.Apply(options);
+            return message;
+        }
+
+        internal PipelineMessage CreateUpdateDogAsAnimalRequest(BinaryContent content, RequestOptions options)
+        {
+            ClientUriBuilder uri = new ClientUriBuilder();
+            uri.Reset(_endpoint);
+            uri.AppendPath("/animals/dog/as-animal", false);
+            PipelineMessage message = Pipeline.CreateMessage(uri.ToUri(), "PUT", PipelineMessageClassifier200);
+            PipelineRequest request = message.Request;
+            request.Headers.Set("Content-Type", "application/json");
+            request.Headers.Set("Accept", "application/json");
+            request.Content = content;
+            message.Apply(options);
+            return message;
+        }
+    }
+}

docs/samples/client/csharp/SampleService/SampleClient/src/Generated/AnimalOperations.cs

@@ -0,0 +1,275 @@
+// <auto-generated/>
+
+#nullable disable
+
+using System;
+using System.ClientModel;
+using System.ClientModel.Primitives;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace SampleTypeSpec
+{
+    /// <summary> The AnimalOperations sub-client. </summary>
+    public partial class AnimalOperations
+    {
+        private readonly Uri _endpoint;
+
+        /// <summary> Initializes a new instance of AnimalOperations for mocking. </summary>
+        protected AnimalOperations()
+        {
+        }
+
+        /// <summary> Initializes a new instance of AnimalOperations. </summary>
+        /// <param name="pipeline"> The HTTP pipeline for sending and receiving REST requests and responses. </param>
+        /// <param name="endpoint"> Service endpoint. </param>
+        internal AnimalOperations(ClientPipeline pipeline, Uri endpoint)
+        {
+            _endpoint = endpoint;
+            Pipeline = pipeline;
+        }
+
+        /// <summary> The HTTP pipeline for sending and receiving REST requests and responses. </summary>
+        public ClientPipeline Pipeline { get; }
+
+        /// <summary>
+        /// [Protocol Method] Update a pet as an animal
+        /// <list type="bullet">
+        /// <item>
+        /// <description> This <see href="https://aka.ms/azsdk/net/protocol-methods">protocol method</see> allows explicit creation of the request and processing of the response for advanced scenarios. </description>
+        /// </item>
+        /// </list>
+        /// </summary>
+        /// <param name="content"> The content to send as the body of the request. </param>
+        /// <param name="options"> The request options, which can override default behaviors of the client pipeline on a per-call basis. </param>
+        /// <exception cref="ArgumentNullException"> <paramref name="content"/> is null. </exception>
+        /// <exception cref="ClientResultException"> Service returned a non-success status code. </exception>
+        /// <returns> The response returned from the service. </returns>
+        public virtual ClientResult UpdatePetAsAnimal(BinaryContent content, RequestOptions options = null)
+        {
+            try
+            {
+                System.Console.WriteLine("Entering method UpdatePetAsAnimal.");
+                Argument.AssertNotNull(content, nameof(content));
+
+                using PipelineMessage message = CreateUpdatePetAsAnimalRequest(content, options);
+                return ClientResult.FromResponse(Pipeline.ProcessMessage(message, options));
+            }
+            catch (Exception ex)
+            {
+                System.Console.WriteLine($"An exception was thrown in method UpdatePetAsAnimal: {ex}");
+                throw;
+            }
+            finally
+            {
+                System.Console.WriteLine("Exiting method UpdatePetAsAnimal.");
+            }
+        }
+
+        /// <summary>
+        /// [Protocol Method] Update a pet as an animal
+        /// <list type="bullet">
+        /// <item>
+        /// <description> This <see href="https://aka.ms/azsdk/net/protocol-methods">protocol method</see> allows explicit creation of the request and processing of the response for advanced scenarios. </description>
+        /// </item>
+        /// </list>
+        /// </summary>
+        /// <param name="content"> The content to send as the body of the request. </param>
+        /// <param name="options"> The request options, which can override default behaviors of the client pipeline on a per-call basis. </param>
+        /// <exception cref="ArgumentNullException"> <paramref name="content"/> is null. </exception>
+        /// <exception cref="ClientResultException"> Service returned a non-success status code. </exception>
+        /// <returns> The response returned from the service. </returns>
+        public virtual async Task<ClientResult> UpdatePetAsAnimalAsync(BinaryContent content, RequestOptions options = null)
+        {
+            try
+            {
+                System.Console.WriteLine("Entering method UpdatePetAsAnimalAsync.");
+                Argument.AssertNotNull(content, nameof(content));
+
+                using PipelineMessage message = CreateUpdatePetAsAnimalRequest(content, options);
+                return ClientResult.FromResponse(await Pipeline.ProcessMessageAsync(message, options).ConfigureAwait(false));
+            }
+            catch (Exception ex)
+            {
+                System.Console.WriteLine($"An exception was thrown in method UpdatePetAsAnimalAsync: {ex}");
+                throw;
+            }
+            finally
+            {
+                System.Console.WriteLine("Exiting method UpdatePetAsAnimalAsync.");
+            }
+        }
+
+        /// <summary> Update a pet as an animal. </summary>
+        /// <param name="animal"></param>
+        /// <param name="cancellationToken"> The cancellation token that can be used to cancel the operation. </param>
+        /// <exception cref="ArgumentNullException"> <paramref name="animal"/> is null. </exception>
+        /// <exception cref="ClientResultException"> Service returned a non-success status code. </exception>
+        public virtual ClientResult<Animal> UpdatePetAsAnimal(Animal animal, CancellationToken cancellationToken = default)
+        {
+            try
+            {
+                System.Console.WriteLine("Entering method UpdatePetAsAnimal.");
+                Argument.AssertNotNull(animal, nameof(animal));
+
+                ClientResult result = UpdatePetAsAnimal(animal, cancellationToken.CanBeCanceled ? new RequestOptions { CancellationToken = cancellationToken } : null);
+                return ClientResult.FromValue((Animal)result, result.GetRawResponse());
+            }
+            catch (Exception ex)
+            {
+                System.Console.WriteLine($"An exception was thrown in method UpdatePetAsAnimal: {ex}");
+                throw;
+            }
+            finally
+            {
+                System.Console.WriteLine("Exiting method UpdatePetAsAnimal.");
+            }
+        }
+
+        /// <summary> Update a pet as an animal. </summary>
+        /// <param name="animal"></param>
+        /// <param name="cancellationToken"> The cancellation token that can be used to cancel the operation. </param>
+        /// <exception cref="ArgumentNullException"> <paramref name="animal"/> is null. </exception>
+        /// <exception cref="ClientResultException"> Service returned a non-success status code. </exception>
+        public virtual async Task<ClientResult<Animal>> UpdatePetAsAnimalAsync(Animal animal, CancellationToken cancellationToken = default)
+        {
+            try
+            {
+                System.Console.WriteLine("Entering method UpdatePetAsAnimalAsync.");
+                Argument.AssertNotNull(animal, nameof(animal));
+
+                ClientResult result = await UpdatePetAsAnimalAsync(animal, cancellationToken.CanBeCanceled ? new RequestOptions { CancellationToken = cancellationToken } : null).ConfigureAwait(false);
+                return ClientResult.FromValue((Animal)result, result.GetRawResponse());
+            }
+            catch (Exception ex)
+            {
+                System.Console.WriteLine($"An exception was thrown in method UpdatePetAsAnimalAsync: {ex}");
+                throw;
+            }
+            finally
+            {
+                System.Console.WriteLine("Exiting method UpdatePetAsAnimalAsync.");
+            }
+        }
+
+        /// <summary>
+        /// [Protocol Method] Update a dog as an animal
+        /// <list type="bullet">
+        /// <item>
+        /// <description> This <see href="https://aka.ms/azsdk/net/protocol-methods">protocol method</see> allows explicit creation of the request and processing of the response for advanced scenarios. </description>
+        /// </item>
+        /// </list>
+        /// </summary>
+        /// <param name="content"> The content to send as the body of the request. </param>
+        /// <param name="options"> The request options, which can override default behaviors of the client pipeline on a per-call basis. </param>
+        /// <exception cref="ArgumentNullException"> <paramref name="content"/> is null. </exception>
+        /// <exception cref="ClientResultException"> Service returned a non-success status code. </exception>
+        /// <returns> The response returned from the service. </returns>
+        public virtual ClientResult UpdateDogAsAnimal(BinaryContent content, RequestOptions options = null)
+        {
+            try
+            {
+                System.Console.WriteLine("Entering method UpdateDogAsAnimal.");
+                Argument.AssertNotNull(content, nameof(content));
+
+                using PipelineMessage message = CreateUpdateDogAsAnimalRequest(content, options);
+                return ClientResult.FromResponse(Pipeline.ProcessMessage(message, options));
+            }
+            catch (Exception ex)
+            {
+                System.Console.WriteLine($"An exception was thrown in method UpdateDogAsAnimal: {ex}");
+                throw;
+            }
+            finally
+            {
+                System.Console.WriteLine("Exiting method UpdateDogAsAnimal.");
+            }
+        }
+
+        /// <summary>
+        /// [Protocol Method] Update a dog as an animal
+        /// <list type="bullet">
+        /// <item>
+        /// <description> This <see href="https://aka.ms/azsdk/net/protocol-methods">protocol method</see> allows explicit creation of the request and processing of the response for advanced scenarios. </description>
+        /// </item>
+        /// </list>
+        /// </summary>
+        /// <param name="content"> The content to send as the body of the request. </param>
+        /// <param name="options"> The request options, which can override default behaviors of the client pipeline on a per-call basis. </param>
+        /// <exception cref="ArgumentNullException"> <paramref name="content"/> is null. </exception>
+        /// <exception cref="ClientResultException"> Service returned a non-success status code. </exception>
+        /// <returns> The response returned from the service. </returns>
+        public virtual async Task<ClientResult> UpdateDogAsAnimalAsync(BinaryContent content, RequestOptions options = null)
+        {
+            try
+            {
+                System.Console.WriteLine("Entering method UpdateDogAsAnimalAsync.");
+                Argument.AssertNotNull(content, nameof(content));
+
+                using PipelineMessage message = CreateUpdateDogAsAnimalRequest(content, options);
+                return ClientResult.FromResponse(await Pipeline.ProcessMessageAsync(message, options).ConfigureAwait(false));
+            }
+            catch (Exception ex)
+            {
+                System.Console.WriteLine($"An exception was thrown in method UpdateDogAsAnimalAsync: {ex}");
+                throw;
+            }
+            finally
+            {
+                System.Console.WriteLine("Exiting method UpdateDogAsAnimalAsync.");
+            }
+        }
+
+        /// <summary> Update a dog as an animal. </summary>
+        /// <param name="animal"></param>
+        /// <param name="cancellationToken"> The cancellation token that can be used to cancel the operation. </param>
+        /// <exception cref="ArgumentNullException"> <paramref name="animal"/> is null. </exception>
+        /// <exception cref="ClientResultException"> Service returned a non-success status code. </exception>
+        public virtual ClientResult<Animal> UpdateDogAsAnimal(Animal animal, CancellationToken cancellationToken = default)
+        {
+            try
+            {
+                System.Console.WriteLine("Entering method UpdateDogAsAnimal.");
+                Argument.AssertNotNull(animal, nameof(animal));
+
+                ClientResult result = UpdateDogAsAnimal(animal, cancellationToken.CanBeCanceled ? new RequestOptions { CancellationToken = cancellationToken } : null);
+                return ClientResult.FromValue((Animal)result, result.GetRawResponse());
+            }
+            catch (Exception ex)
+            {
+                System.Console.WriteLine($"An exception was thrown in method UpdateDogAsAnimal: {ex}");
+                throw;
+            }
+            finally
+            {
+                System.Console.WriteLine("Exiting method UpdateDogAsAnimal.");
+            }
+        }
+
+        /// <summary> Update a dog as an animal. </summary>
+        /// <param name="animal"></param>
+        /// <param name="cancellationToken"> The cancellation token that can be used to cancel the operation. </param>
+        /// <exception cref="ArgumentNullException"> <paramref name="animal"/> is null. </exception>
+        /// <exception cref="ClientResultException"> Service returned a non-success status code. </exception>
+        public virtual async Task<ClientResult<Animal>> UpdateDogAsAnimalAsync(Animal animal, CancellationToken cancellationToken = default)
+        {
+            try
+            {
+                System.Console.WriteLine("Entering method UpdateDogAsAnimalAsync.");
+                Argument.AssertNotNull(animal, nameof(animal));
+
+                ClientResult result = await UpdateDogAsAnimalAsync(animal, cancellationToken.CanBeCanceled ? new RequestOptions { CancellationToken = cancellationToken } : null).ConfigureAwait(false);
+                return ClientResult.FromValue((Animal)result, result.GetRawResponse());
+            }
+            catch (Exception ex)
+            {
+                System.Console.WriteLine($"An exception was thrown in method UpdateDogAsAnimalAsync: {ex}");
+                throw;
+            }
+            finally
+            {
+                System.Console.WriteLine("Exiting method UpdateDogAsAnimalAsync.");
+            }
+        }
+    }
+}

docs/samples/client/csharp/SampleService/SampleClient/src/Generated/DogOperations.RestClient.cs

@@ -0,0 +1,31 @@
+// <auto-generated/>
+
+#nullable disable
+
+using System.ClientModel;
+using System.ClientModel.Primitives;
+
+namespace SampleTypeSpec
+{
+    /// <summary></summary>
+    public partial class DogOperations
+    {
+        private static PipelineMessageClassifier _pipelineMessageClassifier200;
+
+        private static PipelineMessageClassifier PipelineMessageClassifier200 => _pipelineMessageClassifier200 = PipelineMessageClassifier.Create(stackalloc ushort[] { 200 });
+
+        internal PipelineMessage CreateUpdateDogAsDogRequest(BinaryContent content, RequestOptions options)
+        {
+            ClientUriBuilder uri = new ClientUriBuilder();
+            uri.Reset(_endpoint);
+            uri.AppendPath("/dogs/dog/as-dog", false);
+            PipelineMessage message = Pipeline.CreateMessage(uri.ToUri(), "PUT", PipelineMessageClassifier200);
+            PipelineRequest request = message.Request;
+            request.Headers.Set("Content-Type", "application/json");
+            request.Headers.Set("Accept", "application/json");
+            request.Content = content;
+            message.Apply(options);
+            return message;
+        }
+    }
+}