KAFKA-17554: Flaky testFutureCompletionOutsidePoll in ConsumerNetworkClientTest

[m1a2st]

Jira: https://issues.apache.org/jira/browse/KAFKA-17554
In the previous workflow, the test passes under two conditions:

1. The t1 thread is waiting for the main thread's client.wakeup(). If successful, t1 will wake up t2, allowing t2 to complete the future.
2. If t1 fails to receive the client.wakeup() from the main thread, t2 will be woken up by the main thread.
   The flaky test error occurs because neither t1 nor t2 are being properly woken up by the main thread. To resolve this, I suggest adjusting the test flow so that the main thread calls client.wakeup() before the t1 thread starts polling. Then, after t1 starts polling, the t2 thread can begin its poll. Additionally, I propose increasing the wait time in MockClient#maybeAwaitWakeup from 5 seconds to 10 seconds for better reliability.

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)