fix StreamStringTermsAggregatorTests by replacing RandomIndexWriter with plain IndexWriter #18956 WIP

[mkhludnev]

• I replaced RandomIndexWriter to plain IndexWriter to avoid unexpected segments in test which are unable to handle more than single segment
• for other test I've added explicit segmenting. TODO It needs to be reviewed.
• Perhaps StreamingTermsAggregator should reject second segment via assert or there should be a proper publish/reset flow. TODO, WIP

so far this commit fixes the known reason for flackines (I belive)

Description

[Describe what this change achieves]

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

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

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[mkhludnev]

Got reproducible failure on the single segment. It doesn't comply with the current hypothesis.

REPRODUCE WITH: ./gradlew 'null' --tests 'org.opensearch.search.aggregations.bucket.terms.StreamStringTermsAggregatorTests.testReduceSingleAggregation' -Dtests.seed=71C9C5CB31106539 -Dtests.locale=ee -Dtests.timezone=ACT -Druntime.java=21

java.lang.AssertionError: 
Expected: <3>
     but: was <1>
Expected :<3>
Actual   :<1>
<Click to see difference>

	at __randomizedtesting.SeedInfo.seed([71C9C5CB31106539:9E6BEB57758E488D]:0)
	at org.opensearch.search.aggregations.bucket.terms.StreamStringTermsAggregatorTests.testReduceSingleAggregation(StreamStringTermsAggregatorTests.java:```

[github-actions]

❕ Gradle check result for b5cd364: UNSTABLE

Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.93%. Comparing base (6fdb010) to head (b76c2a2).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #19404      +/-   ##
============================================
+ Coverage     72.90%   72.93%   +0.02%     
+ Complexity    69915    69859      -56     
============================================
  Files          5675     5675              
  Lines        320841   320865      +24     
  Branches      46387    46388       +1     
============================================
+ Hits         233909   234011     +102     
+ Misses        68031    67843     -188     
- Partials      18901    19011     +110     

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[rishabhmaurya]

@mkhludnev do you want to address TODO as part of different PR? If yes, we can probably merge this one

[mkhludnev]

fixed.

testReduceSingleAggregation
newIndexWriterConfig() sets MaxDocsBuffered randomly and caused index segmentation. Replaced it with newIndexWriterConfig()

address TODO as part of different PR? If yes, we can probably merge this one

Right. We can treat this PR as urgent fix for reducing CI noize.

[github-actions]

❌ Gradle check result for bca5be7: 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 b76c2a2: SUCCESS

[mkhludnev]

TODO as part of different PR?

attempt #19416

[mkhludnev]

this PR left RandomIndexWriter in testBuildAggregationsBatchWithCountOrder and testBuildAggregationsBatchReset which made'em prone to segmentation. I've fixed these tests in #19416