[fix][test] Fix Mockito stubbing race in TopicListServiceTest

[Denovo1998]

Fixes #25226

Main Issue: #xyz

PIP: #xyz

Motivation

TopicListServiceTest#testCommandWatchUpdateRetries could intermittently fail with a NullPointerException and Mockito UnfinishedStubbingException due to a race between async watcher callbacks and Mockito stubbing.

In particular, the test was setting up doAnswer().when(pulsarCommandSender)... after starting async work, so another thread could invoke methods on the same mock while Mockito was still in “stubbing in progress” state. This made the test flaky and caused occasional false failures.

Modifications

• Move the sendWatchTopicListUpdate stubbing to happen before handleWatchTopicList(...) starts any asynchronous processing.
• Add a short comment to document why the ordering is important (avoid concurrent Mockito stubbing state).

Verifying this change

• Make sure that the change passes the CI checks.

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

Matching PR in forked repository

PR in forked repository: Denovo1998#20

[lhotari]

LGTM, thanks for fixing!

[lhotari]

/pulsarbot rerun-failure-checks