refactor: Simplify metadata serialization by removing redundant JSON conversion

- Remove _make_json_serializable method entirely
- Refactor _convert_value_to_proto to handle dictionaries and lists recursively
- Fix null value handling to use struct_pb2.NULL_VALUE
- Improve code efficiency and maintainability
- All tests passing

Author: sven-cy-yu

src/a2a/utils/proto_utils.py

@@ -60,9 +60,7 @@ def metadata(
     def _convert_value_to_proto(cls, value: Any) -> struct_pb2.Value:
         """Convert Python value to protobuf Value."""
         if value is None:
-            proto_value = struct_pb2.Value()
-            proto_value.null_value = 0
-            return proto_value
+            return struct_pb2.Value(null_value=struct_pb2.NULL_VALUE)
         if isinstance(value, bool):
             return struct_pb2.Value(bool_value=value)
         if isinstance(value, int):
@@ -74,29 +72,22 @@ def _convert_value_to_proto(cls, value: Any) -> struct_pb2.Value:
         if isinstance(value, str):
             return struct_pb2.Value(string_value=value)
         if isinstance(value, dict):
-            serializable_dict = cls._make_json_serializable(value)
-            json_data = json.dumps(serializable_dict, ensure_ascii=False)
-            struct_value = struct_pb2.Struct()
-            json_format.Parse(json_data, struct_value)
-            return struct_pb2.Value(struct_value=struct_value)
+            return struct_pb2.Value(
+                struct_value=struct_pb2.Struct(
+                    fields={
+                        k: cls._convert_value_to_proto(v)
+                        for k, v in value.items()
+                    }
+                )
+            )
         if isinstance(value, list | tuple):
-            list_value = struct_pb2.ListValue()
-            for item in value:
-                converted_item = cls._convert_value_to_proto(item)
-                list_value.values.append(converted_item)
-            return struct_pb2.Value(list_value=list_value)
+            return struct_pb2.Value(
+                list_value=struct_pb2.ListValue(
+                    values=[cls._convert_value_to_proto(item) for item in value]
+                )
+            )
         return struct_pb2.Value(string_value=str(value))
 
-    @classmethod
-    def _make_json_serializable(cls, value: Any) -> Any:
-        if value is None or isinstance(value, str | int | float | bool):
-            return value
-        if isinstance(value, dict):
-            return {k: cls._make_json_serializable(v) for k, v in value.items()}
-        if isinstance(value, list | tuple):
-            return [cls._make_json_serializable(item) for item in value]
-        return str(value)
-
     @classmethod
     def part(cls, part: types.Part) -> a2a_pb2.Part:
         if isinstance(part.root, types.TextPart):