fix: implement RowIdMeta.asdict() and from_dict() for stable row ID serialization

[pengw0048]

Fixes #6403

FragmentMetadata.to_json() raised NotImplementedError when row_id_meta was present (enable_stable_row_ids=True), because PyRowIdMeta.asdict() was stubbed out. This also broke FragmentMetadata.from_json() for the round-trip, since RowIdMeta(**dict) doesn't work on a PyO3 class.

Repro

import lance, pyarrow as pa

uri = "/tmp/repro.lance"
ds = lance.write_dataset(pa.table({"x": [1, 2, 3]}), uri, enable_stable_row_ids=True)
ds.get_fragments()[0].metadata.to_json()  # NotImplementedError: PyRowIdMeta.asdict is not yet supported.

Fix

• Implement PyRowIdMeta.asdict() via pythonize (Rust struct → Python dict)
• Add PyRowIdMeta.from_dict() via depythonize (Python dict → Rust struct)
• Update FragmentMetadata.from_json() to use RowIdMeta.from_dict() instead of RowIdMeta(**dict)
• Add JSON round-trip test to the existing test_fragment_metadata_pickle parametrized test