fix(query): stop flight exchange task after shutdown

[dqhl76]

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Summary

root cause: WatchNotify is sticky; calling notified() again would spin forever.

flowchart TD
    A["ExchangeSinkBuffer / PingPongExchange drop"] --> B["shutdown.notify_waiters()"]
    B --> C["PingPong task: finished = true"]
    C --> D["callback.has_pending()"]
    D --> E["recreate shutdown.notified()"]
    E --> F["sticky notification Ready immediately"]
    F --> C

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fix: after finished signal, only drain the steam not wait notified future again.

close: #20228

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Explain why

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

AI assistance

• AI usage: analyze the GDB/perf data, identify the root cause, implement the fix
• Responsible human: @dqhl76
• The responsible human has read every line of this diff and can explain each change

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This change is 

[dqhl76]

The CI failure is unrelated to this PR. The same test case also fails on the main branch.