feat(submit): configurable confirm retries and idempotent confirm

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.

spark_executor/models.py

@@ -87,6 +87,7 @@ class Connection(BaseModel):
 class PendingSubmission(BaseModel):
     pending_id: str
     app_name: str
+
     connection: str
     master: str
     deploy_mode: str
@@ -103,3 +104,4 @@ class PendingSubmission(BaseModel):
     error: str | None = None
     job_id: str | None = None
     application_id: str | None = None
+    tracking_url: str | None = None