feat(python): add type-safe expression builder API (#3150)

Introduces col(), lit(), func(), and Expr class as alternatives to raw
SQL strings in .where() and .select(). Expressions are backed by
DataFusion's Expr AST and serialized to SQL for remote table compat.

Resolves: 
- https://github.com/lancedb/lancedb/issues/3044 (python api's)
- https://github.com/lancedb/lancedb/issues/3043 (support for filter)
- https://github.com/lancedb/lancedb/issues/3045 (support for
projection)

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/src/python/python.md

@@ -36,6 +36,20 @@ is also an [asynchronous API client](#connections-asynchronous).
 
 ::: lancedb.table.Tags
 
+## Expressions
+
+Type-safe expression builder for filters and projections. Use these instead
+of raw SQL strings with [where][lancedb.query.LanceQueryBuilder.where] and
+[select][lancedb.query.LanceQueryBuilder.select].
+
+::: lancedb.expr.Expr
+
+::: lancedb.expr.col
+
+::: lancedb.expr.lit
+
+::: lancedb.expr.func
+
 ## Querying (Synchronous)
 
 ::: lancedb.query.Query