fix!: return a JSON string instead of dict to be consistent with type…

… annotation

python/python/lance/fragment.py

@@ -65,7 +65,7 @@ def __eq__(self, other: object) -> bool:
 
     def to_json(self) -> str:
         """Serialize :class:`FragmentMetadata` to a JSON blob"""
-        return json.loads(self._metadata.json())
+        return self._metadata.json()
 
     @staticmethod
     def from_json(json_data: str) -> FragmentMetadata: