feat(python): expose register_table_function for Paimon UDTFs

[shyjsarah]

Purpose

Linked issue: close #xxx

paimon-datafusion provides register_vector_search / register_full_text_search to register table-valued functions (UDTFs) on a session. But the Python binding PySQLContext only exposed register_catalog / set_current_* /register_batch / sql — there was no way to reach register_udtf from Python, so these UDTFs were entirely unusable from pypaimon.

This PR exposes a single registration entry point to the Python binding.

Brief change log

• bindings/python/src/context.rs: add SQLContext.register_table_function(name, default_database=None). A single dispatch method (rather than one method per function) keeps the Python API surface stable — it matches on the function name, currently handling vector_search and full_text_search, and raises a clear ValueError for an unknown name. The function is bound to the current catalog.
• crates/integrations/datafusion/src/sql_context.rs: change SQLContext::current_catalog from private to pub. The binding needs the registered Arc<dyn Catalog> to pass to register_*; exposing the accessor lets it read from SQLContext instead of keeping a duplicate catalog handle.
• bindings/python/Cargo.toml: enable the fulltext feature on paimon-datafusion (pulls in tantivy + tempfile, both pure-Rust) so register_full_text_search is compiled into the binding.

Once register_referenced_files_size / register_physical_files_size land on main, wiring them is a two-line addition to the match — the Python signature does not change.

Tests

bindings/python/tests/test_datafusion.py — 5 new tests:

• vector_search / full_text_search register without error
• the optional default_database keyword is accepted
• an unknown function name raises a clear error
• calling before any catalog is registered raises

Registration touches neither the Lumina nor the Tantivy runtime, so the tests are deterministic and need no index fixtures.

API and Format

• New Python API: SQLContext.register_table_function.
• New public Rust API: SQLContext::current_catalog (previously private).
• Build: the binding now enables paimon-datafusion/fulltext (adds tantivy).
• No storage format change.

Documentation

New Python-facing API. The Rust-side docs/src/sql.md already documents the underlying register_* functions; the pypaimon-facing docs live in the apache/paimon repo and can be updated as a follow-up.