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

The issue points to the Marvin agent sample in the a2a-samples repository, specifically the Python code in samples/python/agents/marvin. The task is to update this agent to use the new A2A Python SDK instead of the deprecated common/ directory. Start by examining the current implementation and the error related to TextPart validation. Check the A2A Python SDK documentation for the correct way to structure text parts. Success means the agent runs without the validation error using the new SDK.

由索引模型根據 Issue 內容生成。