[Server][Conformance] Tool with sampling gets stuck

[chr-hertel]

Scenario tools-call-sampling gets stuck for unknown reason.

Reproduce:

make conformance-server

npx @modelcontextprotocol/conformance server --url http://localhost:8000 --scenario tools-call-sampling

[CodeWithKyrian]

The reason is known actually. The StreamableHttpTransport uses SSE for server-client communication. So for sampling to work, the server sends a sampling request via the open SSE stream, the client processes the request and sends the response via a separate HTTP POST and the server needs to receive this POST while still streaming SSE.

PHP's built-in server (which was used in the conformance tests) is single-threaded and a single process so it can only handle one request at a time. While the SSE connection is open, it cannot accept the second request thus a deadlock.

Currently working on a fix and will put up a PR soon.