Support W3C string serialization of traceparent in msgpack formatters

[AArnott]

Closes #1444

[Copilot]

Pull request overview

This PR adds an option to serialize the W3C traceparent field as a plain string (instead of StreamJsonRpc’s compact binary MessagePack encoding) for both MessagePackFormatter and NerdbankMessagePackFormatter, and adds tests asserting the chosen serialization token type.

Changes:

• Add TraceParentAsW3CString init-only option to both MessagePack formatters and route traceparent serialization through string/binary-specific converters/formatters.
• Refactor TraceParent to expose a fixed serialized length constant and a WriteTo(Span<char>) helper.
• Add unit tests validating the on-the-wire MessagePack token type for traceparent under both settings.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
test/StreamJsonRpc.Tests/NerdbankMessagePackFormatterTests.cs	Adds a theory asserting traceparent serializes as Array vs String based on TraceParentAsW3CString.
test/StreamJsonRpc.Tests/MessagePackFormatterTests.cs	Adds the same serialization-format assertion for MessagePackFormatter.
src/StreamJsonRpc/Protocol/TraceParent.cs	Refactors string parsing/serialization helpers; introduces Length and WriteTo.
src/StreamJsonRpc/Polyfills.cs	Consolidates polyfills and adds Encoding.TryGetChars extension for non-NET TFMs.
src/StreamJsonRpc/PolyfillMethods.cs	Removes now-redundant polyfill type (moved into Polyfills).
src/StreamJsonRpc/NerdbankMessagePackFormatter.TraceParentConverter.cs	Splits traceparent converter into binary vs string variants.
src/StreamJsonRpc/NerdbankMessagePackFormatter.cs	Adds TraceParentAsW3CString option and uses it for traceparent serialization.
src/StreamJsonRpc/MessagePackFormatter.cs	Adds TraceParentAsW3CString option, new traceparent formatters, and delegates traceparent serialization through them.

Comments suppressed due to low confidence (2)

src/StreamJsonRpc/NerdbankMessagePackFormatter.cs:488

• Deserialization of traceparent/tracestate is still format-dependent: by default TraceParentConverter expects the compact binary array format, so incoming W3C strings (the scenario in #1444) will still fail unless the receiver opts into TraceParentAsW3CString. If this PR is meant to close #1444, these reads should accept both array and string encodings by inspecting reader.NextMessagePackType (and similarly accept string tracestate).

                else if (TraceParentPropertyName.TryRead(ref reader))
                {
                    TraceParent traceParent = formatter.TraceParentConverter.Read(ref reader, context);
                    result.TraceParent = traceParent.ToString();
                }
                else if (TraceStatePropertyName.TryRead(ref reader))
                {
                    result.TraceState = ReadTraceState(ref reader, context);
                }

src/StreamJsonRpc/MessagePackFormatter.cs:1547

• JsonRpcRequest deserialization uses TraceParentFormatter/ReadTraceState unconditionally, so default settings still reject W3C string-encoded traceparent/tracestate (the interoperability issue described in #1444). If this PR closes #1444, these reads should accept both array and string encodings by switching on reader.NextMessagePackType; keep TraceParentAsW3CString as a serialization preference.

                else if (TraceParentPropertyName.TryRead(stringKey))
                {
                    TraceParent traceParent = this.formatter.TraceParentFormatter.Deserialize(ref reader, options);
                    result.TraceParent = traceParent.ToString();
                }
                else if (TraceStatePropertyName.TryRead(stringKey))
                {
                    result.TraceState = ReadTraceState(ref reader, options);
                }

[drewnoakes]

Thanks for fixing this so quickly Andrew.

LGTM. Could add some tests covering parsing failures.