[chore]: Add more tests for ProtoUtils

The ProtoUtils class contains classes for converting gRPC classes and spec classes.

We have a test class called ToProtoTest that has a very limited set of tests for this.

This issue is to add more test cases to thoroughly test ProtoUtils.ToProto and ProtoUtils.FromProto.

The corresponding tests in the Python SDK can be seen here: https://github.com/a2aproject/a2a-python/blob/main/tests/utils/test_proto_utils.py

We should add more test cases similar to what's in the Python tests.