This commit adds support for request cancellation and tracking of
in-flight requests in the MCP protocol implementation. The key
architectural changes are:
1. Request Lifecycle Management:
- Added _in_flight dictionary to BaseSession to track active requests
- Requests are tracked from receipt until completion/cancellation
- Added proper cleanup via on_complete callback
2. Cancellation Support:
- Added CancelledNotification handling in _receive_loop
- Implemented cancel() method in RequestResponder
- Uses anyio.CancelScope for robust cancellation
- Sends error response on cancellation
3. Request Context:
- Added request_ctx ContextVar for request context
- Ensures proper cleanup after request handling
- Maintains request state throughout lifecycle
4. Error Handling:
- Improved error propagation for cancelled requests
- Added proper cleanup of cancelled requests
- Maintains consistency of in-flight tracking
This change enables clients to cancel long-running requests and
servers to properly clean up resources when requests are cancelled.
Github-Issue:#88
Pydantic treats fields starting with underscore as private/hidden. To fix this,
we need to use Field with alias='_meta' to properly handle these fields while
keeping the external API unchanged. This fixes#103 where meta fields were not
being properly assigned in request contexts.
