Fix flaky HybridQueryExplainIT explanation test via per-_id assertions

[martin-gaievski]

Description

Fixed flaky test for hybrid query explanation processor. Switched to new assert per-document explanation trees keyed by _id (routing is deterministic; only tied-hit order isn't), preserving all structure/score checks and dropping only the Lucene-internal child-count couplings.

Related Issues

#1899

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[github-actions]

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🧪 PR contains tests

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[github-actions]

PR Code Suggestions ✨

Explore these optional code suggestions:

Category	Suggestion	Impact
General	Validate expected document IDs are present After building explanationById, assert that it contains exactly the three expected IDs ("1", "2", "3"). Otherwise, if a document is missing or an unexpected _id appears in the hits, the subsequent assertNotNull checks will still pass on the wrong entries or fail with a less informative message. src/test/java/org/opensearch/neuralsearch/query/HybridQueryExplainIT.java [328-331] Map<String, Map<String, Object>> explanationById = new HashMap<>(); for (Map<String, Object> oneHit : hitsNestedList) { explanationById.put((String) oneHit.get("_id"), getValueByKey(oneHit, "_explanation")); } +assertEquals(Set.of("1", "2", "3"), explanationById.keySet()); Suggestion importance[1-10]: 4 __ Why: Adding an assertion on the keyset would produce a clearer failure message if hits are missing or unexpected, but the existing assertNotNull checks on each explanationById.get(...) already catch missing documents. It's a minor test robustness improvement.	Low

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.43%. Comparing base (a5e4e8f) to head (8746220).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1900      +/-   ##
============================================
- Coverage     83.46%   83.43%   -0.03%     
+ Complexity     3893     3892       -1     
============================================
  Files           291      291              
  Lines         13835    13835              
  Branches       2300     2300              
============================================
- Hits          11547    11543       -4     
- Misses         1455     1458       +3     
- Partials        833      834       +1     

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[martin-gaievski]

"Check Workflow Events" CI action will keep failing unless #1901 is merged, there is a bug in repo infra yaml file