Make RequestIdSTJsonConverter public for AOT/trimming compatibility

Problem

When using SystemTextJsonFormatter with custom JsonSerializerOptions that use TypeInfoResolverChain (the AOT-safe approach), RequestId serialization fails at runtime because:

1. RequestIdSTJsonConverter is internal, so consumers cannot reference it
2. [JsonSerializable(typeof(RequestId))] fails with SYSLIB1220 because RequestId has [JsonConverter(typeof(RequestIdSTJsonConverter))] and that converter is inaccessible (CS0122)
3. MassageUserDataSerializerOptions adds the converter via options.Converters.Add(), but in AOT mode without a reflection-based fallback resolver, Converters alone cannot produce JsonTypeInfo<RequestId> — the converter is unreachable

This surfaces as a NotSupportedException when StandardCancellationStrategy tries to serialize a $/cancelRequest notification containing a RequestId.

Suggested Fix

Make RequestIdSTJsonConverter public so consumers can either:

• Use [JsonSerializable(typeof(RequestId))] in their source-generated contexts
• Reference the converter directly when building custom IJsonTypeInfoResolver implementations

Additionally, MassageUserDataSerializerOptions could be updated to add entries to TypeInfoResolverChain (not just Converters), so the formatter is self-sufficient in AOT mode even when users provide custom options.

Workaround

We currently duplicate the converter logic and wire it via a custom IJsonTypeInfoResolver + JsonMetadataServices.CreateValueInfo<RequestId>(). See: https://github.com/github/copilot-sdk/pull/783

Related

• #840 — RequestId AOT crash (Newtonsoft-based, same class of problem)
• #1230 — AOT NotSupportedException with IAsyncEnumerable structs (same root cause pattern: internal converters + AOT)

cc @AArnott

Start by inspecting RequestIdSTJsonConverter and MassageUserDataSerializerOptions, then trace how StandardCancellationStrategy serializes RequestId with custom TypeInfoResolverChain options. Compare the proposed public-converter approach with the resolver workaround in copilot-sdk PR 783. Done means the selected AOT/trimming path is implemented and RequestId serialization works without a reflection-based fallback.