Fix flaky tests in RemoteStoreStatsIT

[ashking94]

Description

This PR addresses flakiness in remote store download stats correctness tests by improving test reliability and consistency.

Problem

The existing remote store download stats tests (testDownloadStatsCorrectnessSinglePrimarySingleReplica and testDownloadStatsCorrectnessSinglePrimaryMultipleReplicaShards) were experiencing intermittent failures due to:

• Race conditions in stats retrieval
• Inconsistent replication state
• Lack of robust waiting mechanisms

Solution

• Added waitForReplication() method to ensure consistent replication state
• Improved assertion logic to handle potential timing variations
• Enhanced logging and error tracking
• Refactored common test logic to reduce code duplication
• Improved stats validation methods

Related Issues

Resolves #14310

Check List

• Functionality includes testing.
• [ ] API changes companion pull request created, if applicable.
• [ ] Public documentation issue/PR created, if applicable.

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[github-actions]

❌ Gradle check result for 12a112b: 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 12a112b: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 72.43%. Comparing base (342c645) to head (12a112b).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
...rch/index/remote/RemoteSegmentTransferTracker.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #17535      +/-   ##
============================================
+ Coverage     72.40%   72.43%   +0.03%     
- Complexity    65683    65708      +25     
============================================
  Files          5311     5311              
  Lines        304890   304891       +1     
  Branches      44213    44213              
============================================
+ Hits         220743   220850     +107     
+ Misses        66045    65997      -48     
+ Partials      18102    18044      -58     

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[shourya035]

Thanks for the fix @ashking94 🙌

[ashking94]

Before this change, the tests testDownloadStatsCorrectnessSinglePrimarySingleReplica and testDownloadStatsCorrectnessSinglePrimaryMultipleReplicaShards used to fail within 200 iterations. After this change, I have run both the tests for around 3K iterations and they have not failed yet. There are still running, will update here if they fail.