fix(client): send elicitation on originating response stream

[jstar0]

Fixes #557

Motivation and Context

The tool handler in this scenario sends elicitation/create with server.request(), which does not associate it with the active tools/call request. The transport therefore writes it on a separate GET stream instead of the POST response stream consumed by the client. Using extra.sendRequest() correlates the elicitation with its initiating tool call.

How Has This Been Tested?

• Added an HTTP/SSE regression test that asserts elicitation/create arrives on the originating tools/call POST response stream.
• npm test: 49 files and 642 tests passed.
• npm run build, npm run lint, and npm run typecheck passed.
• Ran scenario elicitation-sep1034-client-defaults against the Rust SDK reference conformance client: 5/5 checks passed. The client received the elicitation request and returned the tool result on the same tools/call POST SSE stream.

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

[pkg-pr-new]

Open in StackBlitz

npx https://pkg.pr.new/@modelcontextprotocol/conformance@561

commit: e234a2f

[zhoutk]

Confirming this fixes the failure I reported in #557. The root cause matches what I observed — server.request() on a server without an associated request was delivered on the standalone GET stream, while my client (and the rust-sdk fixture) only read the request's POST response stream. Thanks @jstar0. I maintain the mcp-cpp conformance client fixture (one of the two failing clients in #557); I'll remove the elicitation-sep1034-client-defaults entry from our conformance/expected-failures.yaml and re-baseline against the next suite release.