New JsonValue derived class for JSON primitives

[PranavSenthilnathan]

Adds a new derived class that JsonValueConverter can use when the value is a primitive. Previously JsonElementOfValue was being used and more recently JsonValue. There are downsides with both. The JsonElement-backed class incurs overhead for the JsonDocument it needs to create and JsonValue does not allow common conversions in TryGet. The new class in this PR solves both by only storing necessary information and allowing conversions. It also preserves original escaping in WriteTo.

Fixes #116730

[Copilot]

Pull Request Overview

This PR introduces a new JsonValueOfJsonPrimitive class to optimize handling of JSON primitive values by storing raw UTF-8 bytes and escaping info, refactors reader and converter logic to use it, and adds comprehensive tests for primitive behaviors and conversions.

• Add JsonValueOfJsonPrimitive to store only needed data for primitives and preserve original escaping in WriteTo.
• Update JsonValueConverter, JsonDocument, and Utf8JsonReader to delegate GUID parsing to a new JsonReaderHelper.TryGetValue helper.
• Extend tests in JsonValueTests.cs to cover deserialization, TryGetValue, WriteTo, deep-cloning, and conversion for all primitive types using a new WrappedT<T> wrapper.

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/JsonNode/JsonValueTests.cs	Added tests for primitive JSON values (string, bool, number), WriteTo, cloning, deep-equals, and conversion; introduced WrappedT<T> helper.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Node/JsonValueConverter.cs	Replaced token-type switch with instantiation of JsonValueOfJsonPrimitive for primitives.
src/libraries/System.Text.Json/src/System/Text/Json/Reader/Utf8JsonReader.TryGet.cs	Replaced inline GUID parsing logic with JsonReaderHelper.TryGetValue call.
src/libraries/System.Text.Json/src/System/Text/Json/Reader/JsonReaderHelper.cs	Added new overload TryGetValue(ReadOnlySpan<byte>, bool, out Guid) to centralize GUID parsing.
src/libraries/System.Text.Json/src/System/Text/Json/Nodes/JsonValueOfJsonPrimitive.cs	New internal sealed class implementing optimized storage and parsing for JSON primitives.
src/libraries/System.Text.Json/src/System/Text/Json/Document/JsonDocument.cs	Switched inline GUID parsing in TryGetValue to JsonReaderHelper.TryGetValue.
src/libraries/System.Text.Json/src/System.Text.Json.csproj	Included JsonValueOfJsonPrimitive.cs in project file.

[PranavSenthilnathan]

@eiriktsarpalis I can also take this PR one step further by adding separate derived classes for JSON string and numbers, and introducing the JsonNumber struct like we discussed offline.

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[PranavSenthilnathan]

Keeping this fix more targeted by matching the logic in JsonValueOfElement.

[eiriktsarpalis]

Thanks!

[PranavSenthilnathan]

/backport to release/10.0-preview7

[github-actions]

Started backporting to release/10.0-preview7: https://github.com/dotnet/runtime/actions/runs/16439281496