When asyncio.sleep() fires just before Task.cancel() is called, CPython
sets _must_cancel=True but cannot cancel the already-completed sleep
future, so CancelledError is delivered at the next await (handle_message)
rather than at the sleep. By that point the superseded task has already
popped the merged event from _pending_text_batches, so the superseding
task sees an empty batch and silently drops the message.
Fix: add a synchronous task-registry check between the sleep and the pop.
No await between the check and the pop means no other coroutine can
interleave, so the guard is race-free.
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