Parent: #3401

Summary

Python FTS index creation currently rejects field names containing ., which prevents valid Lance nested field paths such as payload.text.

Scope

• Replace the top-level-only Python FTS guard with Lance field path validation.
• Support nested text fields in sync and async Python FTS APIs.
• Cover match, phrase, and hybrid search on a nested text field.
• Keep clear errors for unsupported list/struct shapes that are not valid text leaves.

Acceptance criteria

• create_fts_index("payload.text") succeeds for a nested string field.
• match, phrase, and hybrid search can target the nested text field.
• Existing negative tests are updated to reject only invalid field paths or invalid field types.