[Bug]: Agent card for A2A Protocol version 0.3.0 optional fields

### What happened?

I am running the test suite against a Dart SDK, see this [issue](https://github.com/shamblett/a2a/issues/74) using this command
```
./run_tck.py --sut-host http://localhost:9999 --transport jsonrpc
```

I am hitting several errors where according to the V0.3.0 of the A2A Protocol specification the agent card fields are optional or don't exist. an example is ```Failed: Agent card declares no supportedInterfaces — cannot determine which transports to test```. Unless I've missed something there is no 'supportedInterfaces' field on the V0.3 agent card. The HTML report attached to the issue shows the complete story. log output below shows some errors.

What protocol versions of the A2A specification does the toolkit support? Can I select the version?

### Relevant log output

```shell
agent_card: $.capabilities.extensions: None is not of type 'array'
agent_card: $.capabilities.pushNotifications: None is not of type 'boolean'
agent_card: $.documentationUrl: None is not of type 'string'
agent_card: $.iconUrl: None is not of type 'string'
agent_card: $.securitySchemes: None is not of type 'object'
agent_card: $.signatures: None is not of type 'array'
agent_card: Missing required fields: {'supportedInterfaces'}
```

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct