Add hybrid search support for DataFusion

[JingsongLi]

Summary

Adds hybrid search support to paimon-rust by combining multiple search routes with Paimon-compatible rankers. Exposes a Spark-style hybrid_search table function in the DataFusion integration; vector-only hybrid search works without the fulltext feature, while full-text routes still require it.

Changes

• Add core HybridSearchBuilder with vector routes, optional full-text routes, and rrf, weighted_score, and mrr ranking strategies.
• Extend vector and full-text builders to return scored search results for hybrid ranking.
• Add DataFusion hybrid_search(...) table function using Spark-style array(named_struct(...)) route definitions.
• Register hybrid_search by default with DataFusion catalogs; keep full_text_search and full-text hybrid route execution behind the fulltext feature.
• Document DataFusion hybrid search usage, vector-only examples, rankers, and full-text feature requirements.
• Add coverage for vector-only hybrid search with multiple vector routes.

Testing

• cargo fmt --all
• cargo test -p paimon hybrid_search_builder -- --nocapture
• cargo test -p paimon --features fulltext hybrid_search_builder -- --nocapture
• cargo test -p paimon-datafusion --test read_tables hybrid_search_tests::test_hybrid_search_multiple_vector_routes_spark_shape -- --nocapture
• cargo test -p paimon-datafusion --features fulltext --test read_tables hybrid_search_tests::test_hybrid_search_multiple_vector_routes_spark_shape -- --nocapture
• cargo test -p paimon --features fulltext full_text_search_builder -- --nocapture
• cargo test -p paimon-datafusion --features fulltext --test read_tables fulltext_tests::test_full_text_search_paimon -- --nocapture
• cargo test -p paimon-datafusion --test read_tables vector_search_tests::test_vector_search_java_vindex_table -- --nocapture
• cargo test -p paimon vector_search::tests::test_vector_search_clone_preserves_include_row_ids -- --nocapture
• cargo clippy -p paimon --lib -- -D warnings
• cargo clippy -p paimon --features fulltext --lib -- -D warnings
• cargo clippy -p paimon-datafusion --lib --tests -- -D warnings
• cargo clippy -p paimon-datafusion --features fulltext --lib --tests -- -D warnings

Notes

The DataFusion table function currently returns the matched table rows. Exposing a Spark-compatible metadata score column can be added separately once scan output metadata columns are supported.

[leaves12138]

LGTM. Reviewed the hybrid search builder, DataFusion table function parsing/execution path, fulltext feature gate, docs, and tests. Verified locally with targeted unit/integration checks and fulltext feature compilation.