[BugFix] Reject outer WHERE on vectorSearch() subqueries to prevent silently dropped filters

[mengweieric]

Summary

Fixes the P0 tracker item Outer WHERE on subquery silently dropped from DSL: when vectorSearch() is wrapped in a subquery and the outer query has its own WHERE clause, the outer predicate never reaches the push-down contract. The plan shape after analysis is LogicalFilter over LogicalProject over the scan builder, so the PUSH_DOWN_FILTER rule (which matches filter directly above scanBuilder) does not fire. The filter is then applied in memory after k-NN has already returned top-k documents ranked by vector distance, which can silently yield zero rows with no explanation.

Failing shape

SELECT * FROM (SELECT v.firstname, v.state
               FROM vectorSearch(table='idx', field='emb',
                                 vector='[1,2]', option='k=5') AS v) t
WHERE t.state = 'TX'

Fix

Per the tracker (either reject vectorSearch() inside subqueries with outer WHERE, or document clearly) this PR chooses rejection as the safer preview behavior.

• Adds a post-optimization validatePlan(LogicalPlan root) hook on TableScanBuilder (default no-op). Scan builders can now inspect their ancestors once all push-down rules have settled.
• Wires it into Planner.plan() so every scan builder in the optimized plan gets a chance to validate.
• VectorSearchIndexScanBuilder overrides the hook to walk the plan and reject the outer-WHERE-over-subquery shape: a LogicalFilter whose descendant chain reaches this scan builder through one or more LogicalProject nodes (the subquery-boundary marker). A filter directly above the scan builder (WHERE on vectorSearch() itself) is explicitly allowed because that path has already been exercised by push-down.
• Error message names the shape and recommends moving the predicate inside the subquery so it is applied during the k-NN search.

Test plan

• Unit tests in VectorSearchIndexScanBuilderTest covering the bad shape, nested subqueries, and three positive controls (filter directly on scanBuilder, inner filter wrapped in outer project, no filter at all) plus a bare-scanBuilder defensive baseline.
• Integration tests in a new VectorSearchSubqueryIT covering the exact tracker query shape, the with-LIMIT variant, the _explain path, and positive controls for inner WHERE and no-WHERE subqueries.
• Regression: VectorSearchIT (18/18) and VectorSearchExplainIT (11/11) still green.
• ./gradlew spotlessCheck clean.
• ./gradlew :core:test and :opensearch:test green.

[mengweieric]

Closing and reopening against upstream opensearch-project/sql.