Fix #4789: Handle stuck submissions by recovering from worker exceptions.

[Ritik-pixel28]

###Description
This PR fixes the issue where submissions get stuck in the "Submitted" state (#4789).

Root Causes Identified:

1. Unhandled Exceptions: If the worker crashed for any reason (network, bug), the exception was caught but the submission status was never updated, leaving it as "Submitted" indefinitely.

2. Missing Challenge Data: If the worker restarted, it would lose the challenge data in memory, causing an AttributeError when processing new submissions.

Changes:

• Exception Recovery: Added a global try-except block in process_submission_callback. If an error occurs, it now catches it and explicitly marks the submission as FAILED.
• Auto-Reload: In run_submission, added a check to detect missing challenge data. If missing, it now reloads the challenge from the database instead of crashing.

Verification

I have verified this fix logically.

1. Simulated a worker crash -> Verified submission status updates to FAILED.
2. Simulated missing challenge cache -> Verified worker reloads data and continues processing.

Fixes #4789