test: Add numeric string roundtrip test and fix mypy type error

- Add test case for numeric strings (e.g., zip codes, numeric IDs) in metadata edge cases
- Fix mypy type error in _convert_value_to_proto for null_value assignment
- Use proper struct_pb2.NullValue.NULL_VALUE instead of integer 0
- Ensure numeric strings remain as strings after roundtrip conversion

Author: sven-cy-yu

src/a2a/utils/proto_utils.py

@@ -71,7 +71,7 @@ def _make_dict_serializable(cls, value: Any) -> Any:
     def _convert_value_to_proto(cls, value: Any) -> struct_pb2.Value:
         if value is None:
             proto_value = struct_pb2.Value()
-            proto_value.null_value = 0
+            proto_value.null_value = struct_pb2.NullValue.NULL_VALUE
             return proto_value
 
         if isinstance(value, bool):

tests/utils/test_proto_utils.py

@@ -342,6 +342,7 @@ def test_metadata_edge_cases(self):
             'large_number': 9999999999999999,
             'negative_number': -42,
             'float_precision': 0.123456789,
+            'numeric_string': '12345',
         }
 
         # Convert to proto and back
@@ -358,3 +359,4 @@ def test_metadata_edge_cases(self):
         assert roundtrip_metadata['large_number'] == 9999999999999999
         assert roundtrip_metadata['negative_number'] == -42
         assert abs(roundtrip_metadata['float_precision'] - 0.123456789) < 1e-10
+        assert roundtrip_metadata['numeric_string'] == '12345'