fix: add metadata to task in proto_utils.py

Author: sokoliva

src/a2a/utils/proto_utils.py

@@ -203,6 +203,7 @@ def task(cls, task: types.Task) -> a2a_pb2.Task:
                 if task.history
                 else None
             ),
+            metadata=cls.metadata(task.metadata),
         )
 
     @classmethod