Marvin Agent - 503 Errors

### What happened?

@daavoo @zzstoatzz The Sample agent at https://github.com/google-a2a/a2a-samples/tree/main/samples/python/agents/marvin currently uses the code in the `common/` directory which is deprecated. Please update this agent to use the A2A Python SDK https://github.com/google/a2a-python

```
| pydantic_core._pydantic_core.ValidationError: 1 validation error for TextPart
| text
| Input should be a valid string [type=string_type, input_value=[TextPart(type='text', te...ioned.', metadata=None)], input_type=list]
| For further information visit https://errors.pydantic.dev/2.11/v/string_type
```
### Code of Conduct

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