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.

In the previous implementation, we used a CountDownLatch to control the execution of three threads, but it often led to race conditions. Currently, we have modified it to use two threads to test this scenario.

I run I=0; while ./gradlew :clients:test --tests ConsumerNetworkClientTest.testFutureCompletionOutsidePoll --rerun --fail-fast; do (( I=$I+1 )); echo "Completed run: $I"; sleep 1; done and pass 3000+ times.

Committer Checklist (excluded from commit message)

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