fix: support JSON serialization of FragmentMetadata with stable row IDs

`FragmentMetadata.to_json()` raised `NotImplementedError` when
`row_id_meta` was present (i.e. `enable_stable_row_ids=True`), because
`PyRowIdMeta.asdict()` was not implemented.

Implement `asdict()` using `pythonize` to convert the Rust enum directly
to a Python dict. Also fix `from_json()` to deserialize `row_id_meta`
via `RowIdMeta.from_json()` instead of attempting `RowIdMeta(**dict)`.

Author: pengw0048

python/python/lance/fragment.py

@@ -143,7 +143,7 @@ def from_json(json_data: str) -> FragmentMetadata:
 
         row_id_meta = json_data.get("row_id_meta")
         if row_id_meta is not None:
-            row_id_meta = RowIdMeta(**row_id_meta)
+            row_id_meta = RowIdMeta.from_json(json.dumps(row_id_meta))
 
         created_at_version_meta = json_data.get("created_at_version_meta")
         if created_at_version_meta is not None:

python/python/tests/test_fragment.py

@@ -470,11 +470,18 @@ def test_fragment_metadata_pickle(tmp_path: Path, enable_stable_row_ids: bool):
     if enable_stable_row_ids:
         assert frag_meta.row_id_meta is not None
 
-    # Pickle and unpickle the fragment metadata
+    # Pickle round-trip
     round_trip = pickle.loads(pickle.dumps(frag_meta))
-
     assert frag_meta == round_trip
 
+    # JSON round-trip
+    json_data = frag_meta.to_json()
+    json_round_trip = FragmentMetadata.from_json(json.dumps(json_data))
+    assert frag_meta.id == json_round_trip.id
+    assert frag_meta.physical_rows == json_round_trip.physical_rows
+    if enable_stable_row_ids:
+        assert json_round_trip.row_id_meta is not None
+
 
 def test_deletion_file_with_base_id_serialization():
     """Test that DeletionFile with base_id serializes correctly."""

python/src/fragment.rs

@@ -627,10 +627,10 @@ pub struct PyRowDatasetVersionMeta(pub RowDatasetVersionMeta);
 
 #[pymethods]
 impl PyRowIdMeta {
-    fn asdict(&self) -> PyResult<Bound<'_, PyDict>> {
-        Err(PyNotImplementedError::new_err(
-            "PyRowIdMeta.asdict is not yet supported.s",
-        ))
+    fn asdict(&self, py: Python<'_>) -> PyResult<PyObject> {
+        pythonize::pythonize(py, &self.0)
+            .map(|b| b.unbind())
+            .map_err(|err| PyValueError::new_err(format!("Could not convert RowIdMeta: {}", err)))
     }
 
     pub fn json(&self) -> PyResult<String> {