Serializer: Fixes unsafe stream cast in FromStream<T>

[NaluTripician]

Description

Fixes #5620

Replaces the unsafe (T)(object)stream cast pattern with safe is pattern matching in all FromStream<T> serializer implementations across the SDK.

Problem

The FromStream<T> method in multiple serializer implementations uses the following pattern:

if (typeof(Stream).IsAssignableFrom(typeof(T)))
{
    return (T)(object)stream;
}

typeof(Stream).IsAssignableFrom(typeof(T)) returns true when T is Stream or any subclass (e.g., MemoryStream, FileStream). If T is a specific Stream subclass but the runtime stream parameter is a different Stream type, the cast (T)(object)stream throws a raw InvalidCastException with no context about what went wrong.

Example that throws:

// T = MemoryStream, but stream is actually a FileStream at runtime
serializer.FromStream<MemoryStream>(someFileStream); // InvalidCastException!

Fix

Replaced the unsafe cast with safe is pattern matching:

if (typeof(Stream).IsAssignableFrom(typeof(T)))
{
    if (stream is T typedStream)
    {
        return typedStream;
    }

    throw new InvalidCastException(
        $"Stream of type '{stream.GetType().FullName}' is not compatible "
        + $"with the requested type '{typeof(T).FullName}'.");
}

This provides:

• ✅ Safe runtime type checking (no unexpected InvalidCastException)
• ✅ A descriptive error message identifying both the actual and expected types
• ✅ No behavioral change for the common case (T = Stream)

Files Changed

Core SDK Serializers (2):

• Microsoft.Azure.Cosmos/src/Serializer/CosmosJsonDotNetSerializer.cs
• Microsoft.Azure.Cosmos/src/Serializer/CosmosSystemTextJsonSerializer.cs

Sample Code (3):

• Microsoft.Azure.Cosmos.Samples/Usage/SystemTextJson/CosmosSystemTextJsonSerializer.cs
• Microsoft.Azure.Cosmos.Samples/Usage/ReEncryption/ReEncryptionSupport/ReEncryptionJsonSerializer.cs
• Microsoft.Azure.Cosmos.Samples/Usage/ItemManagement/Program.cs

Encryption Modules (2):

• Microsoft.Azure.Cosmos.Encryption/src/CosmosJsonDotNetSerializer.cs
• Microsoft.Azure.Cosmos.Encryption.Custom/src/Common/CosmosJsonDotNetSerializer.cs

Unit Tests (2):

• Microsoft.Azure.Cosmos/tests/Microsoft.Azure.Cosmos.Tests/CosmosJsonSerializerUnitTests.cs — 2 new tests
• Microsoft.Azure.Cosmos/tests/Microsoft.Azure.Cosmos.Tests/Json/CosmosSystemTextJsonSerializerTest.cs — 2 new tests

Not Changed

• PatchOperationCore{T}.cs — Has a similar-looking pattern but casts FROM T TO Stream (upcast), which always succeeds. No fix needed.
• Test utility serializers — Internal test code, not customer-facing.

Testing

• 4 new unit tests added (2 per serializer):
  • ValidateFromStreamWithBaseStreamType / TestFromStreamWithBaseStreamType — Confirms FromStream<Stream>(memoryStream) succeeds (regression test)
  • ValidateFromStreamWithIncompatibleStreamTypeThrowsDescriptiveError / TestFromStreamWithIncompatibleStreamTypeThrowsDescriptiveError — Confirms FromStream<FileStream>(memoryStream) throws InvalidCastException with a descriptive message containing both type names
• All existing tests continue to pass

[NaluTripician]

Addressed both comments — simplified to just if (stream is T typedStream) { return typedStream; } across all files.

Re: 'Is above if still needed?' — Good call, the outer typeof(Stream).IsAssignableFrom(typeof(T)) guard is no longer needed. The stream is T pattern match already handles the type check at runtime.

Re: 'Isn't it supposed to fallback to the copy?' — You're right. Removed the throw and let incompatible cases fall through to the normal deserialization path. The simplified stream is T check handles both concerns cleanly.