Function drain_queue_with_probe 

pub async fn drain_queue_with_probe<R, Fut, P>(
    repo_root: &Path,
    once: bool,
    run_mission: R,
    probe: P,
) -> Result<DrainReport>
where
    R: Fn(String) -> Fut,
    Fut: Future<Output = Result<i32>>,
    P: Fn(&Path, &str) -> Result<ReadinessReport>,

drain_queue with an injectable readiness probe. Engine and host tests use this seam to script proceed / park / rate-limit decisions without shelling out to whichever agent CLIs happen to be installed on the test machine.