Function keep_drain_after_accepted_quiesce_failure 

pub async fn keep_drain_after_accepted_quiesce_failure(
    is_draining: &AtomicBool,
    marker_path: &Path,
    status: StatusCode,
    message: &'static str,
) -> Response

Keep the API drained and persist the marker when a quiesce request was accepted but its outcome is unknown, so a restart stays safe.