Harden confirm_submit_job for unreliable test environments and transient
spark-submit failures:
- Add confirm_max_retries and confirm_retry_delay_seconds settings
(env-configurable).
- SUBMITTED pending returns cached result without re-running spark-submit.
- FAILED pending is reset to PENDING and retried.
- CANCELLED pending is still rejected.
- On success, persist pending as SUBMITTED before creating the in-memory Job
so a job_store failure cannot leave the record as PENDING while the YARN
app is already running.
- Persist tracking_url on PendingSubmission for idempotent returns.
Tests cover retry-then-success, max-retries-exceeded, idempotency,
FAILED reset, and pending-saved-before-job-store.