fix(client): return correct type for foundry#WithOptions

src/Anthropic.Foundry/AnthropicFoundryClient.cs

@@ -26,6 +26,12 @@ public AnthropicFoundryClient(IAnthropicFoundryCredentials azureCredentials)
         BaseUrl = new Uri(url, UriKind.Absolute);
     }
 
+    private AnthropicFoundryClient(IAnthropicFoundryCredentials azureCredentials, ClientOptions options)
+        : base(options)
+    {  
+        _azureCredentials = azureCredentials ?? throw new ArgumentNullException(nameof(azureCredentials));   
+    }
+
     [Obsolete("The {nameof(APIKey)} property is not supported in this configuration.", true)]
 #pragma warning disable CS0809 // Obsolete member overrides non-obsolete member
     public override string? APIKey
@@ -35,6 +41,11 @@ public override string? APIKey
         init => throw new NotSupportedException($"The {nameof(APIKey)} property is not supported in this configuration.");
     }
 
+    public override IAnthropicClient WithOptions(Func<ClientOptions, ClientOptions> modifier)
+    {
+        return new AnthropicFoundryClient(_azureCredentials, modifier(_options));
+    }
+
     protected override ValueTask BeforeSend<T>(HttpRequest<T> request, HttpRequestMessage requestMessage, CancellationToken cancellationToken)
     {
         _azureCredentials.Apply(requestMessage);

src/Anthropic/AnthropicClient.cs

@@ -21,7 +21,7 @@ static Random Random
         get { return _threadLocalRandom.Value!; }
     }
 
-    readonly ClientOptions _options;
+    protected readonly ClientOptions _options;
 
     public HttpClient HttpClient
     {
@@ -65,7 +65,7 @@ public virtual string? AuthToken
         init { this._options.AuthToken = value; }
     }
 
-    public IAnthropicClient WithOptions(Func<ClientOptions, ClientOptions> modifier)
+    public virtual IAnthropicClient WithOptions(Func<ClientOptions, ClientOptions> modifier)
     {
         return new AnthropicClient(modifier(this._options));
     }