feat: expose list-element FTS document granularity (#4050)

## Summary

LanceDB could not request Lance's list-element FTS document granularity
through Python or Remote APIs, and generic nested-field resolution
exposed Arrow's internal `item` segment instead of the public field
path.

This exposes typed `row | list_element` configuration for Python FTS
index creation and match/phrase queries, preserves `_doc_index`, and
keeps nested FTS paths public (for example, `docs.content`). Remote
list-element requests require server API version 0.6.0 so older servers
cannot silently execute them with row semantics; explicit row requests
remain compatible.

## Compatibility

Omitted index and query parameters retain row granularity. Remote row
index creation omits the new wire field.

## Tracking


[ENT-2342](https://linear.app/lancedb/issue/ENT-2342/expose-list-element-fts-document-granularity-end-to-end)

docs/src/python/python.md

@@ -159,6 +159,8 @@ and combined with [BooleanQuery][lancedb.query.BooleanQuery].
 
 ::: lancedb.query.FullTextOperator
 
+::: lancedb.query.DocumentGranularity
+
 ::: lancedb.query.Occur
 
 ## Embeddings