fix(python): preserve nested fields in filtered scans (#3816)

## Summary

- add a Python regression for filtered scans over deeply nested
list/struct values
- verify the public query path returns every nested sibling field
without an Arrow schema mismatch

## Root cause

Older Lance scanner code split filter projections into partial nested
arrays and did not recursively merge List<Struct> children, so a sibling
field could be omitted from the returned array while remaining in the
declared schema. That merge was fixed upstream in
lance-format/lance#4838, and the Lance revision currently pinned by
LanceDB contains that fix. This test makes the repair durable at the
LanceDB Python API boundary.

## Validation

- cd python && uv run --extra tests pytest
python/tests/test_query.py::test_filter_preserves_deeply_nested_struct_fields
-q
- cd python && uv run --extra tests pytest python/tests/test_query.py -q
- uv run --project python --extra tests --extra dev ruff check .
- uv run --project python --extra tests --extra dev ruff format --check
python/python/tests/test_query.py

Fixes #2217

<!-- lance-gatekeeper-fix:v1 agent=9ddc0efd1f13888b2a7a896881c6e68a
generation=1 -->

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

python/python/tests/test_query.py

@@ -833,6 +833,28 @@ def test_with_row_id(table: lancedb.table.Table):
     assert rs["_rowid"].to_pylist() == [0, 1]
 
 
+def test_filter_preserves_deeply_nested_struct_fields(tmp_db):
+    # Regression test for https://github.com/lancedb/lancedb/issues/2217.
+    data = [
+        {
+            "item": "test",
+            "sub_obj": {
+                "second_sub_obj": [
+                    {
+                        "third_sub_obj": {"int_val": 1},
+                        "str_val": "test",
+                    }
+                ]
+            },
+        }
+    ]
+    table = tmp_db.create_table("deeply_nested_struct", data=data)
+
+    result = table.search().where("item = 'test'").limit(1).to_arrow()
+
+    assert result.to_pylist() == data
+
+
 def test_blob_v2_query_omits_auto_row_id(tmp_db):
     table = _create_blob_v2_query_table(tmp_db, "test_blob_v2_omits_auto_rowid")