Retry starts after concurrent job limits

[Silvren]

Summary

• recognize the unofficial ConcurrentJobLimit API error as a retryable job-start outcome
• return affected jobs to the persisted created state instead of marking them start_failed
• re-submit created jobs during a later manager polling cycle, providing non-blocking backoff without sleeping in worker threads
• factor start-task submission into one path so initial starts and retries share authentication and accounting behavior
• add unit and end-to-end coverage for a rejected first start followed by a successful retry and completed job

Testing

• python -m pytest tests/extra/job_management -q -k "not resilient" (179 passed, 6 deselected)
• targeted job-start and worker tests (35 passed)
• pre-commit checks on all changed files

Fixes #838