[fix](variant) Bind Variant search to nested indexes

[eldenmoon]

What problem does this PR solve?

Backport #63660 to branch-4.1.

This fixes Variant inverted-index search binding for scalar and nested Variant paths. It resolves search clauses to the correct scalar or nested Variant index reader, maps nested leaf query results back to the expected document scope, and preserves null bitmap semantics when the truth bitmap is empty.

The backport keeps the original behavior changes and adapts the segment index iterator call to the existing branch-4.1 ColumnReader API.

Issue Number: N/A

Related PR: #63660

Release note

Fix Variant inverted-index search binding for scalar and nested Variant paths.

Check List (For Author)

• Test:
  • Unit Test: ./run-be-ut.sh --run --filter='Variant:FunctionSearchTest.TestBuildLeafQueryDirectUnknownClauseUsesLeafMapper:FunctionSearchNestedTest.*:BitSetQueryTest.EmptyTruthBitmapPreservesNullBitmap'
    • Result: 207 passed, 1 skipped (VariantDocModeCompactionTest.variant_doc_mode_compaction_merge_10_segments, Release-only slow test)
• Behavior changed: Yes. Fixes Variant inverted-index search binding and null bitmap handling.
• Does this need documentation: No

[hello-stephen]

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