Add jittered retries for queue failures

[owenshuo]

Summary

• store scheduled tasks with their queue metadata so delayed retries can be dequeued with the original task payload
• retry transient queue failures with bounded exponential backoff and injectable jitter for deterministic tests
• preserve task identity across retries and record terminal completed/cancelled/failed outcomes to reject stale retry paths

Fixes #3929

Validation

• python3 -m pytest tests/test_scheduler.py tests/test_config.py -q
• python3 -m flake8 src/orchestrator/scheduler.py src/orchestrator/engine.py tests/test_scheduler.py
• git diff --check