test(python): cover nullable fixed-size-list ingestion (#3812)

## Summary

- add regression coverage for adding dictionary rows with a nullable
fixed-size-list column
- verify ordinary list columns remain aligned alongside the null
fixed-size-list value

## Root cause

PyArrow infers an all-`None` dictionary column as the generic `null`
type. The original schema-alignment path treated the target
fixed-size-list type as proof that the inferred source was also
list-like and unconditionally accessed `value_field`, which raised
`AttributeError`. Current alignment logic correctly falls back to the
target type when the source is not list-like; this test locks in that
repair for the reported ingestion path.

## Validation

- `uv run --extra tests pytest
python/tests/test_table.py::test_add_with_empty_fixed_size_list_drops_bad_rows
python/tests/test_table.py::test_add_nullable_fixed_size_list_with_none
python/tests/test_table.py::test_add_nullable_struct_with_none -q`
- `uv run --with pyarrow==19.0.1 --extra tests pytest
python/tests/test_table.py::test_add_nullable_fixed_size_list_with_none
-q`
- `uv run --project python --extra dev ruff format --check
python/python/tests/test_table.py`
- `uv run --project python --extra dev ruff check .`

Fixes #2340

<!-- lance-gatekeeper-fix:v1 agent=cb0475e85e764f79bd03b35eb8955ec4
generation=1 -->

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

python/python/tests/test_table.py

@@ -1845,6 +1845,27 @@ def test_add_with_empty_fixed_size_list_drops_bad_rows(mem_db: DBConnection):
     assert np.allclose(data["embedding"].to_pylist()[0], np.array([0.1] * 16))
 
 
+def test_add_nullable_fixed_size_list_with_none(mem_db: DBConnection):
+    """Regression test for issue #2340."""
+    table = mem_db.create_table(
+        "test_nullable_fixed_size_list",
+        schema=pa.schema(
+            [
+                pa.field("id", pa.string()),
+                pa.field("feature", pa.list_(pa.float32(), 256)),
+                pa.field("tags", pa.list_(pa.string())),
+            ]
+        ),
+    )
+
+    table.add([{"id": "1", "feature": None, "tags": ["tag1", "tag2"]}])
+
+    result = table.to_arrow()
+    assert result.to_pylist() == [
+        {"id": "1", "feature": None, "tags": ["tag1", "tag2"]}
+    ]
+
+
 def test_add_nullable_struct_with_none(mem_db: DBConnection):
     """Regression test for issue #2654: a nullable struct column whose
     first batch contains only None values must not crash in