Fix flaky shared-stream overflow test that times out in CI

[FrankYang0529]

The failure is a race in test_ack_mode_queue_full_during_fanout_does_not_break_iteration, not in the shared-stream code.

The test pre-fills the overflowing subscriber's queue (maxsize 1) with a filler so the poll loop's fan-out put_nowait raises QueueFull and the subscriber is force-failed with _SubscriberOverflow. But it begins draining that subscriber's stream (collect_full) concurrently with the poll loop's fan-out. When the consumer wins the race it pulls the filler off the queue first; the fan-out put_nowait then succeeds, the overflow path is never taken.

The fix makes the overflow deterministic: before draining the subscriber's stream, wait until the poll loop has actually force-failed it (1 in group._failed_subscribers).

closes: #68628

Was generative AI tooling used to co-author this PR?

• Yes (Claude Code with Opus 4.8)

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