fix(scheduler): prevent duplicate session firing and zombie session r…

[theonlychant]

Dealing with the issue [Bug] Scheduled task fires 12x instead of once — duplicate sessions with unique task IDs #289

Description

Fixes two related scheduler bugs affecting Jules scheduled tasks:

Bug 1 - Duplicate session firing: A single scheduled task configured to run once daily was firing 6–13x per trigger, creating duplicate sessions and consuming session quota. Root cause is a backend scheduler idempotency gap the dispatch mechanism retried when it did not receive a timely ACK from the worker pool, creating a new session instead of recognizing the original was already started. Fixed by adding idempotency key tracking so duplicate dispatches for the same scheduled trigger are detected and dropped.

Bug 2 - Zombie session re-triggering: Sessions that had reached "Completed" status were reverting to "Planning" and re-executing ~15–30 minutes later. Fixed by making completed session state immutable once a session reaches "Completed", state transitions back to "Planning" are rejected.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

Test plan

1. Configure a scheduled task to run daily
2. Use a prompt that triggers moderate work (file reads + code analysis)
3. Wait for the scheduled trigger and observe via jules remote list --session
4. Verify exactly 1 session is created per trigger
5. Verify completed sessions remain in "Completed" state and do not revert