[BugFix] Fix FILTER(WHERE) dropped on aggregates in unified SQL path

[dai-chen]

Description

This PR fixes aggregate FILTER(WHERE ...) clauses being silently dropped in the unified SQL path, where the predicate was ignored when building the Calcite AggCall. The filter is now applied to the aggregate call and its referenced columns are preserved through the pre-aggregation trimming projection, with no impact on PPL.

Related Issues

Part of #5248

Check List

• New functionality includes testing.
• New functionality has been documented.
• New functionality has javadoc added.
• New functionality has a user manual doc added.
• New PPL command checklist all confirmed.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff or -s.
• Public documentation issue/PR created.

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PR Reviewer Guide 🔍

(Review updated until commit 874dff8)

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🧪 PR contains tests

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PR Code Suggestions ✨

Explore these optional code suggestions:

Category	Suggestion	Impact
General	Prevent duplicate column references The method getAggregateFilterInputRefs may add duplicate RexInputRef entries when multiple aggregates reference the same column in their filter predicates. Consider using a Set or deduplicating the results to avoid redundant column references in the trimming projection. core/src/main/java/org/opensearch/sql/calcite/CalciteRelNodeVisitor.java [1808-1818] private List<RexInputRef> getAggregateFilterInputRefs( List<UnresolvedExpression> aggExprList, CalcitePlanContext context) { - List<RexInputRef> refs = new ArrayList<>(); + Set<RexInputRef> refs = new LinkedHashSet<>(); for (UnresolvedExpression aggExpr : aggExprList) { AggregateFunction aggFunc = extractAggregateFunction(aggExpr); if (aggFunc != null && aggFunc.condition() != null) { refs.addAll(PlanUtils.getInputRefs(rexVisitor.analyze(aggFunc.condition(), context))); } } - return refs; + return new ArrayList<>(refs); } Suggestion importance[1-10]: 6 __ Why: The suggestion correctly identifies that duplicate RexInputRef entries could be added when multiple aggregates reference the same column in their filter predicates. Using a Set to deduplicate is a valid optimization that improves efficiency, though the code would still function correctly with duplicates. The impact is moderate as it's an optimization rather than a bug fix.	Low

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Persistent review updated to latest commit 874dff8