When Stop was already waiting for turn/completed as the 429 retry
deadline fired, the deadline marked the turn exhausted but left its
containment unset until its own interrupt wait finished. If Stop's
wait expired first, the turn ended as reusable although neither
interrupt was confirmed, so a follow-up could reuse a thread that may
still be retrying. Treat an exhausted turn with a native thread as
uncontained until turn/completed confirms it ended.