fix(client): make the Streamable HTTP no-reply failure diagnosable and cover chunked application/json replies

[ump45nose]

Summary

Addresses the two asks in #1862 for the Streamable HTTP client transport: production-equivalent coverage for a chunked application/json reply, and diagnostics on the no-reply failure.

1. Coverage for a chunked application/json reply (new StreamableHttpResponseCorrelationTests)

The existing coverage for this path uses an in-memory StringContent, which is already buffered and carries a Content-Length. The two new tests drive a raw loopback socket instead, answering with HTTP 200, Content-Type: application/json; charset=utf-8, Transfer-Encoding: chunked, no Content-Length, and the body split across 16-byte chunks — consumed through the transport's normal ResponseHeadersRead flow (the request body is written chunked as well, since JsonContent has no known length). They assert that a correlated JSON-RPC error is (a) delivered to the session and (b) drives the existing server/discover → initialize fallback to 2025-06-18.

Finding worth recording for the original report: both tests pass on current main with no transport change. I could not reproduce the loss described in the issue for a correlated error in a chunked application/json 200 response, including when the server/discover probe is answered with a -32600 error. So the framing itself does not appear to be the cause; the failure mode that remains reachable is a reply the transport cannot correlate.

2. The no-reply failure now reports what actually happened

McpException: Streamable HTTP POST response completed without a reply to request with ID: 1 was the only signal, even when the peer had clearly answered. Because response bodies are only logged at Trace level via LogTransportReceivedMessageSensitive, everything needed to diagnose the failure was invisible at default logging.

The transport now accumulates non-sensitive shape facts about the uncorrelated response and appends them to that message:

• before: ...without a reply to request with ID: 1
• after: ...without a reply to request with ID: 1. The response Content-Type was 'application/json', and the response contained a JSON-RPC response or error with ID '42', which does not match the request ID.

The reported cases are the four requested in the issue plus two cheap neighbours: empty JSON body; JSON-RPC deserialization failure; a message that is not a response/error for this request; a response or error whose ID differs from the request ID; a response or error with a null ID (JSON-RPC permits this when the peer could not determine the request ID); and a text/event-stream that ended with a network error. The response media type is always included.

Only message shape and IDs are reported. No message payload is copied into the exception message, so the existing sensitive-logging boundary is preserved, and the original ...without a reply to request with ID: X prefix is kept intact.

Verification

• dotnet test tests/ModelContextProtocol.Tests -f net10.0 --filter "FullyQualifiedName~StreamableHttpResponseCorrelationTests" → 6 passed, 0 failed.
• Same filter with src/ModelContextProtocol.Core/Client/StreamableHttpClientSessionTransport.cs temporarily restored to HEAD → 4 failed, 2 passed. The four failures are exactly the new diagnostics assertions (NoReply_ResponseWithDifferentId_ExceptionNamesTheObservedId, NoReply_NullIdErrorResponse_ExceptionReportsTheNullId, NoReply_EmptyJsonBody_ExceptionReportsTheEmptyBody, NoReply_NonJsonContentType_ExceptionReportsTheMediaType); the two chunked-response tests pass on unmodified main, which is the finding above.
• dotnet test tests/ModelContextProtocol.Tests -f net10.0 --filter "FullyQualifiedName~Tests.Transport|FullyQualifiedName~Tests.Client|FullyQualifiedName~Tests.Protocol" → 1358 passed, 0 failed, 2 skipped.

Related to #1862.

AI assistance disclosure: AI was used to discover this opportunity and draft the change or text. The submission was checked against the prepared artifact and recorded verification evidence.