Fix flaky DefaultStreamPoller test

[varunbharadwaj]

Description

Fix flaky DefaultStreamPollerTests.testConsumerReinitializationWithNoInitialMessages test. Observed the following error: Unfinished stubbing detected here
Initialize the mocks before starting the poller to avoid race conditions.

Related Issues

Resolves #20069

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[github-actions]

❌ Gradle check result for a4510ec: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

❌ Gradle check result for a4510ec: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

❌ Gradle check result for a4510ec: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

✅ Gradle check result for a4510ec: SUCCESS

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.24%. Comparing base (3ceeffa) to head (a4510ec).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #20075      +/-   ##
============================================
- Coverage     73.29%   73.24%   -0.05%     
+ Complexity    71656    71650       -6     
============================================
  Files          5790     5790              
  Lines        327549   327549              
  Branches      47181    47181              
============================================
- Hits         240061   239928     -133     
- Misses        68261    68419     +158     
+ Partials      19227    19202      -25     

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