[chore]: Add more tests for ProtoUtils

[fjuma]

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.

[Akanksha928]

Hi @fjuma, can I take this up?

[kabir]

@Akanksha928 Thanks, I've assigned you the issue :-)

[Akanksha928]

Hi, just letting you know that I'm still working on this! It's mid term season so I've been a bit busy but I'll have a PR raised by the end of this week :)

[fjuma]

@Akanksha928 Good luck with mid terms!

[fjuma]

Just a reminder, there's only a few days left to go for Hacktoberfest 🎃