Summary

• Remove the _send_response call from RequestResponder.cancel(). The MCP cancellation spec says receivers must not send a response for a cancelled request.
• When send_notification is called with a CancelledNotification, immediately resolve the pending response stream so send_request() returns instead of waiting indefinitely.
• Update a now-stale comment in lowlevel/server.py that said "responder.cancel() already sent an error response."

Motivation and Context

RequestResponder.cancel() was sending ErrorData(code=0, message="Request cancelled") back to the sender after receiving notifications/cancelled. The cancellation spec says receivers SHOULD NOT do this. Claude Code, for example, logs "Received a response for an unknown message ID" and reconnects every time a long-running tool gets cancelled.

The TypeScript SDK already gets this right: the sender rejects its own pending promise locally when it sends the cancellation (source), and the receiver just calls controller.abort() without sending anything back. This PR brings the Python SDK in line with that behavior.

How Has This Been Tested?

• uv run --frozen pytest tests/shared/test_session.py — all 9 tests pass, including test_request_cancellation (unchanged) and the new test_cancel_notification_for_unknown_request_id
• uv run --frozen pytest tests/server/test_cancel_handling.py — all 3 tests pass
• ./scripts/test — 1172 tests, 100% branch coverage

Breaking Changes

Code that inspects e.code on a cancellation MCPError will see -32001 (REQUEST_TIMEOUT) instead of 0. The message "Request cancelled" is unchanged. The TypeScript SDK uses ErrorCode.RequestTimeout for this same case, so the behavior now matches across SDKs.

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)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Fixes #2480