Add tests for process exit code handling in compute local runners

- Add "Fail" function to zentest-appstub that exits with a configurable
  exit code read from the action's Constants, enabling controlled failure
  testing without needing command-line flags
- Fix GetMaxRetriesForQueue to use -1 as sentinel default so that
  max_retries=0 in queue config correctly disables retries
- Add four new test cases:
  - exit_code.failed_action: non-zero exit produces failed queue status
    and succeeded=false in history
  - exit_code.auto_retry: retries exhaust before final failure with
    correct retry_count in history
  - exit_code.reschedule_failed: manual reschedule returns 409 when
    retry limit is reached
  - exit_code.mixed_success_and_failure: queue counters are correct
    when actions succeed and fail in the same queue

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