feat(python): accept expressions in update filters (#3876)

## Summary
- allow Python sync, async, and remote table updates to accept type-safe
`Expr` filters
- serialize expression filters before invoking the existing update
implementation
- cover numeric-looking text and apostrophe-containing text in sync and
async regression tests

## Root cause
`Table.update` was the remaining Python write path that required callers
to construct a raw SQL predicate. Dynamic text interpolated without SQL
literal encoding could therefore be parsed as an integer, float, or
unterminated string instead of Utf8. The expression API already encodes
literals safely for query and delete filters.

## Validation
- `cd python && .venv/bin/pytest
python/tests/test_table.py::test_update_async
python/tests/test_table.py::test_update_expr_filter_literals_async
python/tests/test_table.py::test_update
python/tests/test_table.py::test_update_expr_filter_literals -q`
- `cd python && .venv/bin/pytest python/tests/test_expr.py -q`
- `cd python && .venv/bin/ruff format --check .`
- `cd python && .venv/bin/ruff check .`

Fixes #1869

<!-- lance-gatekeeper-fix:v1 agent=01f1e7b69c65e8b6d3b3c1e1a7918179
generation=1 -->

---------

Co-authored-by: Gatefixer <313497061+lancedb-gatefixer[bot]@users.noreply.github.com>
Co-authored-by: Xuanwo <github@xuanwo.io>

python/src/expr.rs

@@ -130,6 +130,14 @@ impl PyExpr {
 
     // ── utilities ────────────────────────────────────────────────────────────
 
+    /// Return the referenced column name for a bare column expression.
+    fn column_name(&self) -> Option<String> {
+        match &self.0 {
+            DfExpr::Column(column) if column.relation.is_none() => Some(column.name.clone()),
+            _ => None,
+        }
+    }
+
     /// Render the expression as a SQL string (useful for debugging).
     fn to_sql(&self) -> PyResult<String> {
         lancedb::expr::expr_to_sql_string(&self.0).map_err(|e| PyValueError::new_err(e.to_string()))