Serialize sbyte as JSON number in transport and execution writers

[glen-84]

Summary

Follow-up to #9827. That PR added the missing sbyte case to StrawberryShake's JsonSerializationHelper, but as noted on #9825 that helper is not on the v16 HTTP request path, so Byte variables were still serialized as JSON strings.

The Byte scalar's runtime type is sbyte (ByteType : IntegerTypeBase<sbyte>), and the same byte-without-sbyte gap existed in two more JSON value writers:

• Utf8JsonWriterHelper (HotChocolate.Transport.Abstractions) — the actual HTTP request path for StrawberryShake, GraphQLHttpClient, and Fusion subgraph requests. This is what fixes the reported issue.
• JsonValueFormatter (HotChocolate.Execution.Abstractions) — reached when a boxed sbyte is formatted via extensions, Any, or raw dictionaries.

Both now emit sbyte as a JSON number instead of falling through to string serialization.

Test plan

• Added OperationRequestTests.Should_WriteSByteVariableAsNumber — an sbyte variable serializes as 2, not "2", through OperationRequest.WriteTo.
• Added JsonValueFormatterTests.WriteSByteAsNumber — JsonValueFormatter.WriteValue emits sbyte as a number.
• Both confirmed failing before the fix and passing after; Transport.Http.Tests (65) and Execution.Abstractions.Tests (105) pass on net8.0/net9.0/net10.0.

Closes #9825

[Copilot]

Pull request overview

Fixes sbyte (Byte scalar runtime type in v16) being serialized as a JSON string in request/response JSON writers by adding explicit sbyte handling so it is emitted as a JSON number.

Changes:

• Add sbyte handling to Utf8JsonWriterHelper.WriteFieldValue so OperationRequest.WriteTo (HTTP request path) writes sbyte variables as JSON numbers.
• Add sbyte handling to JsonValueFormatter.WriteValue so boxed sbyte values (e.g., extensions/Any/raw dictionaries) format as JSON numbers.
• Add targeted tests covering both writers’ sbyte serialization behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/HotChocolate/Core/src/Execution.Abstractions/Execution/JsonValueFormatter.cs	Adds sbyte case to write boxed sbyte as a JSON number.
src/HotChocolate/AspNetCore/src/Transport.Abstractions/Serialization/Utf8JsonWriterHelper.cs	Adds sbyte case to write transport-layer values as JSON numbers (fixes HTTP request serialization).
src/HotChocolate/Core/test/Execution.Abstractions.Tests/Execution/JsonValueFormatterTests.cs	New unit test asserting JsonValueFormatter writes sbyte as 2 (number).
src/HotChocolate/AspNetCore/test/Transport.Http.Tests/OperationRequestTests.cs	New unit test asserting OperationRequest.WriteTo serializes sbyte variables as numbers.

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