Add PolyTypeJsonFormatter as an experimental API

[AArnott]

This is a formatter intended for use in the vs-servicehub repo to support FrameworkServices so that we can have a full stack brokered service system that is NativeAOT safe.
The formatter is hard to use, and will likely evolve into a fully PolyType-based JSON serializer. In the meantime, I have [Experimental] slapped onto it to avoid having to maintain the public API for any user outside of vs-servicehub.

[Copilot]

Pull Request Overview

This PR adds a new PolyTypeJsonFormatter class that provides NativeAOT-compatible JSON serialization for StreamJsonRpc using PolyType. The formatter is marked as experimental and enables AOT scenarios where dynamic code generation isn't available.

Key changes:

• Introduces PolyTypeJsonFormatter as a new formatter option with PolyType-based serialization
• Refactors RequestIdJsonConverter into a shared RequestIdSTJsonConverter for use across formatters
• Creates supporting infrastructure classes (IGenericTypeArgAssist, SourceGenerationContext)
• Adds comprehensive test coverage with multiple test classes for different scenarios

Reviewed Changes

Copilot reviewed 15 out of 16 changed files in this pull request and generated 26 comments.

Show a summary per file

File	Description
src/StreamJsonRpc/PolyTypeJsonFormatter.cs	New formatter implementation with PolyType support and experimental attribute
src/StreamJsonRpc/RequestIdSTJsonConverter.cs	Extracted RequestId converter for System.Text.Json reuse
src/StreamJsonRpc/SourceGenerationContext.cs	Source generation context for built-in types
src/StreamJsonRpc/IGenericTypeArgAssist.cs	Helper interfaces for generic type argument handling
src/StreamJsonRpc/RequestId.cs	Added System.Text.Json converter attribute
src/StreamJsonRpc/SystemTextJsonFormatter.cs	Refactored to use shared RequestIdSTJsonConverter
src/StreamJsonRpc/StreamJsonRpc.csproj	Whitespace formatting fixes
test files	Test classes for PolyTypeJsonFormatter covering various scenarios

Comments suppressed due to low confidence (2)

src/StreamJsonRpc/PolyTypeJsonFormatter.cs:1347

• This foreach loop implicitly filters its target sequence - consider filtering the sequence explicitly using '.Where(...)'.
  src/StreamJsonRpc/PolyTypeJsonFormatter.cs:1359
• This foreach loop implicitly filters its target sequence - consider filtering the sequence explicitly using '.Where(...)'.

[etvorun]