Cancel Session Task

Cancel an in-flight task for a session.

Handles both lifecycle states uniformly:

• Queued (chatStatus='queued') — the dispatcher hasn't claimed the row yet. Flip the session back to idle and return; no executor cancel event needed.
• Running (chatStatus='running') — publish a cancel event to the executor via RabbitMQ FANOUT, then poll Redis until the task status flips out of running or a 5 s timeout is hit.