fix(index): push down deeply nested scalar filters

[lance-gatekeeper]

Summary

• consume every trailing string literal when extracting paths from variadic get_field expressions
• preserve chained get_field support and canonical nested field formatting
• cover the production planner path for a two-level nested scalar index filter

Root cause

DataFusion optimization combines deeply nested field access into one variadic get_field call. The scalar-index path extractor required exactly two arguments, returned no path for depth two or greater, and left the filter as a full-scan refinement.

Validation

• cargo fmt --all -- --check
• cargo test -p lance-index (925 passed, 2 ignored; 7 doctests passed)
• cargo clippy --all --tests --benches -- -D warnings

Fixes #8170

[lance-gatekeeper]

✅ Gate recommendation: approve.

The change fixes the optimizer-boundary mismatch at the narrow path-extraction boundary while preserving chained access, canonical nested-field formatting, and safe fallback for malformed expressions. This is preferable to broader planner normalization because it covers every expression producer without changing planner or format contracts.

[Xuanwo]

Thank you for this fix!