fix(index): decline incompatible JSON accessor paths

[lance-gatefixer]

Summary

• decline JSON scalar-index routing when a typed accessor path has different semantics from JSONPath
• keep safe bare object-key accessors indexed while rejecting nested, root-prefixed, normalized, and numeric paths
• add parser boundary coverage and an indexed-versus-unindexed regression test

Root cause

JSON index training evaluates the configured path as full JSONPath, but typed accessors perform a single literal object-key lookup or root-array index lookup. The query router matched only the path spelling, so incompatible expressions such as $.user.age could query nested index keys even though the unindexed accessor looked for a literal top-level key.

Validation

• cargo fmt --all
• cargo clippy --all --tests --benches -- -D warnings
• cargo test -p lance-index test_path_matches_typed_accessor
• cargo test -p lance-index test_expressions -- --nocapture
• cargo test -p lance-index test_multi_json_indices_route_by_path
• cargo test -p lance test_nested_json_path_typed_accessor_matches_unindexed_results
• cargo test -p lance test_json_extract_matches_unindexed_results

Fixes #9257

[lance-gatekeeper]

✅ Gate recommendation: approve.

The guard matches the actual semantic boundary: only an unchanged bare object key is shared by JSONPath training and typed-accessor lookup, while root-prefixed, nested, normalized, and numeric spellings now fall back to correct scans. This preserves safe bare-key acceleration without broadening accessor semantics, and the parser/routing plus indexed-versus-unindexed regressions cover the boundary.