Serialize sbyte values as JSON numbers in StrawberryShake

[glen-84]

Summary

• Byte! variables were serialized as JSON strings ("2") instead of numbers (2), causing v16 servers to reject them.
• Root cause: ByteSerializer became ScalarSerializer<sbyte> in v16, but JsonSerializationHelper.WriteValue had no sbyte case, so values fell through to string serialization.
• Added the missing sbyte case (placed beside byte, signed-before-unsigned to match the existing 16/32/64-bit pairs).

Test plan

• Added JsonSerializationHelperTests.WriteSByteAsNumber verifying sbyte serializes as a JSON number.
• Added the previously-missing SByte entry to the comprehensive Serialize_Request_With_Extensions numeric-matrix snapshot test.
• Both tests confirmed failing before the fix and passing after; full Json + Serialization test areas pass (154 tests, net8.0/net9.0/net10.0).

Closes #9825

[Copilot]

Pull request overview

Fixes StrawberryShake JSON variable/extension serialization for Byte! (runtime sbyte in v16) by ensuring sbyte values are emitted as JSON numbers instead of quoted strings, restoring compatibility with v16 GraphQL servers.

Changes:

• Add explicit sbyte handling to JsonSerializationHelper.WriteValue so boxed sbyte values serialize as JSON numbers.
• Add a targeted unit test asserting sbyte serializes to an unquoted JSON number.
• Extend the existing “extensions numeric matrix” snapshot test to include SByte.

Reviewed changes

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

File	Description
src/StrawberryShake/Client/src/Core/Json/JsonSerializationHelper.cs	Adds sbyte case to write numeric JSON output instead of falling back to string serialization.
src/StrawberryShake/Client/test/Core.Tests/Json/JsonSerializationHelperTests.cs	Adds a focused unit test covering sbyte numeric serialization.
src/StrawberryShake/Client/test/Core.Tests/Json/JsonOperationRequestSerializerTests.cs	Extends the extensions serialization test to include an SByte entry.
src/StrawberryShake/Client/test/Core.Tests/Json/__snapshots__/JsonOperationRequestSerializerTests.Serialize_Request_With_Extensions.snap	Updates snapshot to reflect SByte being serialized as a JSON number.

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