Fix flaky testMaxScoreWithSortOnScoreAndCollapsingResults

[bowenlan-amzn]

Description

Fixes a flaky test in QueryPhaseTests.testMaxScoreWithSortOnScoreAndCollapsingResults that fails intermittently with certain random seeds, like A86CC83E4E8823F2.

./gradlew ':server:test' --tests 'org.opensearch.search.query.QueryPhaseTests' \
    -Dtests.method='testMaxScoreWithSortOnScoreAndCollapsingResults' \
    -Dtests.seed=A86CC83E4E8823F2

Example failure: https://build.ci.opensearch.org/job/gradle-check/64947/testReport/junit/org.opensearch.search.query/QueryPhaseTests/testMaxScoreWithSortOnScoreAndCollapsingResults__p0_5_p1_org_opensearch_search_query_ConcurrentQueryPhaseSearcher_1f6ba47b_/

Test came from #19181

The test was making order-dependent assertions on collapsed search results:

assertThat(topDocs.collapseValues[0], equalTo(0L)); // user == 0
assertThat(topDocs.collapseValues[1], equalTo(1L)); // user == 1

However, the order of collapsed results is non-deterministic when documents have equal scores because:

1. All test documents match the query foo:bar with identical scores
2. Documents are indexed with alternating user values (0, 1, 0, 1, ...)
3. RandomIndexWriter may reorder/merge documents during indexing based on the random seed
4. With certain seeds (e.g., A86CC83E4E8823F2), documents with user=1 end up at lower doc IDs than user=0
5. When collapsing on the user field with equal scores, the document encountered first (lower doc ID) is returned
6. This causes the collapse values to be [1, 0] instead of the expected [0, 1]

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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[github-actions]

❕ Gradle check result for 2d9b1d3: 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 73.01%. Comparing base (ac6dfa1) to head (2d9b1d3).

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #19513      +/-   ##
============================================
- Coverage     73.09%   73.01%   -0.08%     
+ Complexity    70553    70484      -69     
============================================
  Files          5716     5716              
  Lines        322926   322926              
  Branches      46770    46770              
============================================
- Hits         236032   235786     -246     
- Misses        67882    68102     +220     
- Partials      19012    19038      +26     

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[bowenlan-amzn]

@gaobinlong please take a look when you got time

[gaobinlong]

Thanks @bowenlan-amzn, by comparing with testCollapseQuerySearchResults(), I found that index sorting is missing in this method, so add an index sorting configuration to the IndexWriter is a better solution?