fix(python): accept Arrow scalars in table updates (#3838)

## Summary
- convert PyArrow scalar values through their Python representation
before SQL literal rendering
- add an end-to-end regression for updating a fixed-size-list vector
from a queried FixedSizeListScalar

## Root cause
Python update literal conversion used single dispatch for native Python
and NumPy values but had no PyArrow Scalar registration. A
FixedSizeListScalar returned by a query therefore reached the
unsupported generic conversion instead of the existing recursive list
converter.

## Validation
- uv run --extra tests pytest python/tests/test_table.py::test_update
python/tests/test_table.py::test_update_with_arrow_scalar
python/tests/test_table.py::test_update_types -q
- uv run --extra tests pytest python/tests/test_util.py -q
- uv run --project python --extra tests --extra dev ruff format --check
python/python/lancedb/util.py python/python/tests/test_table.py
- uv run --project python --extra tests --extra dev ruff check .

Fixes #1228

<!-- lance-gatekeeper-fix:v1 agent=950dd892194e53b61c203d5e3715cac7
generation=1 -->

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

python/python/lancedb/util.py

@@ -395,6 +395,11 @@ def _(value: dict):
     )
 
 
+@value_to_sql.register(pa.Scalar)
+def _(value: pa.Scalar):
+    return value_to_sql(value.as_py())
+
+
 @value_to_sql.register(np.ndarray)
 def _(value: np.ndarray):
     return value_to_sql(value.tolist())

python/python/tests/test_table.py

@@ -2282,6 +2282,20 @@ def test_update(mem_db: DBConnection):
     assert np.allclose(v, np.array([[1.2, 1.9], [1.1, 1.1]]))
 
 
+def test_update_with_arrow_scalar(mem_db: DBConnection):
+    schema = pa.schema({"id": pa.int64(), "vector": pa.list_(pa.float32(), 4)})
+    table = mem_db.create_table("my_table", schema=schema)
+    table.add([{"id": 1, "vector": [1.0, 2.0, 3.0, 4.0]}])
+
+    value = table.search().select(["vector"]).limit(1).to_arrow()["vector"][0]
+    assert isinstance(value, pa.FixedSizeListScalar)
+
+    result = table.update(where="id == 1", values={"vector": value})
+
+    assert result.rows_updated == 1
+    assert table.to_arrow()["vector"].to_pylist() == [[1.0, 2.0, 3.0, 4.0]]
+
+
 def test_update_types(mem_db: DBConnection):
     table = mem_db.create_table(
         "my_table",