KAFKA-20423: Fix flakiness of testWakeupWithFetchDataAvailable

[chickenchickenlove]

Description

I couldn't reproduce it in my local. This PR is based on the specific
assumptions and fix non-deterministic path caused by it.

1. The foreground test thread puts the fetch response into
   MockClient.responses via respondFrom(...).
2. The heartbeat thread enters MockClient.poll() first.
3. The heartbeat thread removes the fetch response from the queue by
   calling responses.poll().
4. However, it gets descheduled before calling response.onComplete().
5. The foreground thread calls client.poll(0, ...), but since the
   response queue is empty, it completes nothing and returns.
6. consumer.wakeup() is called.
7. The first consumer.poll(Duration.ZERO) immediately throws
   WakeupException from maybeTriggerWakeup().
8. consumer.position(tp0) already has a valid position of 0, so it
   returns 0 directly without performing a network poll.
9. The next consumer.poll(Duration.ZERO) also returns empty because
   the response is not yet in pendingCompletion and not in the
   FetchBuffer either.
10. After that, the heartbeat thread wakes up and calls
    response.onComplete(), but by then the assertion has already
    failed.

Although I couldn't reproduce it in my local, this scenario make sense
to me. Given this, this PR applies the change, and I plan to monitor
the CI trend afterward to confirm whether the flakiness is resolved.

Reviewers: Lianet Magrans lmagrans@confluent.io

[chickenchickenlove]

@lianetm Hi!
When you get a chance, could you take a look?

[github-actions]

A label of 'needs-attention' was automatically added to this PR in order to raise the
attention of the committers. Once this issue has been triaged, the triage label
should be removed to prevent this automation from happening again.

[lianetm]

Thanks @chickenchickenlove ! Makes sense to me. Could you rebase pls? (build failure, seems it's just that we are missing this fa097fa ?)

[chickenchickenlove]

@lianetm
Thanks for your comments!
I rebased it and CI worked well!
When you get a chance, please take a look 🙇‍♂️

[lianetm]

Thanks! LGTM