Make RequestIdSTJsonConverter public for AOT/trimming compatibility

[PureWeen]

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: github/copilot-sdk#783

Related

• Error getting value from 'RequestId' on net6.0-ios app #840 — RequestId AOT crash (Newtonsoft-based, same class of problem)
• Using a struct in an IAsyncEnumerable parameter with native AOT doesn't work #1230 — AOT NotSupportedException with IAsyncEnumerable structs (same root cause pattern: internal converters + AOT)

cc @AArnott