A stop racing CapturedCDP.call() can close the supervisor loop before the
queued _send runs; run_coroutine_threadsafe's future then never resolves and
fut.result(timeout=None) hangs the consumer thread forever. Poll the unbounded
wait and raise CapturedCDPInvalid once the loop is closed. Fold the salvaged
validation tests into the dispatch file (one fixture, no F811), and tighten
them: the deadline test stalls inside the result grace, socket close asserts
CapturedCDPInvalid, the async validator asserts the refusal message.
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