fix: send JSONRPCError instead of bare exceptions in streamable HTTP client

[Kludex]

Summary

• When the streamable HTTP client encounters errors (unexpected content type, JSON parse failure, SSE parse failure), it sends bare Exception objects that never resolve the pending send_request() — causing the caller to hang until timeout.
• Send JSONRPCError with the request's id so the response stream unblocks immediately, following the pattern already used by the 404 handler.
• Fixes Cannot handle Unexpected content type when initializing MCP session #1382, fixes streamable_http client call_tool hangs when receiving invalid JSONRPCMessage #1144.

Test plan

• test_unexpected_content_type_sends_jsonrpc_error — server returns text/plain, client gets MCPError immediately (no timeout)
• test_invalid_json_response_sends_jsonrpc_error — server returns invalid JSON body, client gets MCPError immediately
• uv run --frozen ruff check passes
• uv run --frozen pyright passes

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

[Copilot]

Pull request overview

This PR fixes a hang in the Streamable HTTP client by ensuring transport-layer failures (unexpected Content-Type, JSON parse errors, SSE parse errors) are surfaced to the session as JSONRPCError responses tied to the original request id, allowing send_request() to resolve immediately instead of waiting for a timeout.

Changes:

• Emit JSONRPCError (with request id) for unexpected response Content-Type and JSON body parse failures in the Streamable HTTP client.
• Emit JSONRPCError (with request id) when an SSE message event can’t be parsed, unblocking the pending request.
• Add tests covering unexpected content types and invalid JSON responses for non-SDK servers.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
src/mcp/client/streamable_http.py	Converts previously “bare exception sent to stream” cases into JSONRPCError responses keyed by request id.
src/mcp/types/jsonrpc.py	Aligns JSONRPCError.id typing with RequestId.
tests/client/test_notification_response.py	Reworks test server setup to ASGITransport and adds regression tests for immediate MCPError on bad responses.

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[felixweinberger]

Thanks for the fix!

nit: no test for the SSE parse failure path (lines 167-175), though it's pragma: no cover and follows the same pattern as the two tested paths. Not blocking.

looks like CI failures are just about coverage checks. ready to stamp once that's fixed