test: update specs for nullable fetch and correct deleteOn/retry semantics

- Add non-null assertion (!) to fetch() call sites where the record is
  expected to exist, reflecting the now-nullable return type.
- Add maxAttempts: 1 to deleteOn: failure and deleteOn: always queues so
  the first failure is terminal and deleteOn still applies.
- Add "should not delete a job that has retries remaining" to verify the
  fix: row survives intermediate failures and is deleted only after
  exhausting maxAttempts.
- Add "should emit success even when worker returns void" to cover the
  void-worker success-event regression.
- Attach dequeue listeners before queue.start() in the retry test to
  avoid missing the first event.