[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.