When /new is issued while an agent is actively processing, the confirmation response was never sent to the user because cancel_session_processing() was called before _send_with_retry(). Task cancellation side effects could silently drop the response.
Fix: reorder to send the response BEFORE cancelling the old task. Add logging at the send point (matching the pattern at line 2800 in _process_message_background) so future failures are visible.
Closes: #18912