fix(scan): respect family-specific global-index search modes

[XiaoHongbo-Hope]

Purpose

Rust scans only honored the legacy global-index.search-mode option. Paimon's family-specific options therefore had no effect: for example, scalar-index.search-mode=full silently used FAST mode and could omit rows outside current scalar-index coverage.

Changes

• Resolve scalar, vector, and full-text search modes with the family option taking precedence and global-index.search-mode remaining as the compatibility fallback.
• Use the corresponding family mode in scalar scans, vector search, and full-text/hybrid search.
• Add a scalar FULL regression that builds a BTree index, appends matching unindexed rows, and verifies both planning ranges and returned rows.
• Exercise family-specific vector and full-text options in existing fallback tests.

Tests

• cargo test -p paimon spec::core_options::tests -- --nocapture (66 passed)
• cargo test -p paimon table::btree_global_index_build_builder::tests -- --nocapture (29 passed)
• Targeted vector, full-text, and hybrid search-mode tests
• cargo clippy -p paimon --all-targets --features fulltext -- -D warnings
• cargo fmt --all -- --check
• git diff --check

[QuakeWang]

LGTM

[JingsongLi]

+1