Travel Planner Agent: cannot import name 'AgentAuthentication' from 'a2a.types'

### What happened?

Following the instructions for [travel_planner_agent](https://github.com/google-a2a/a2a-samples/blob/main/samples/python/agents/travel_planner_agent/README.md), step 3 (`uv run .`) fails with the following error:
> ImportError: cannot import name 'AgentAuthentication' from 'a2a.types'

### Relevant log output

```shell
Using CPython 3.13.3
Creating virtual environment at: .venv
Built travel-planner @ file:///path/to/a2a-samples/samples/python/agents/travel_planner_agent
Installed 65 packages in 261ms
/path/to/a2a-samples/samples/python/agents/travel_planner_agent/.venv/lib/python3.13/site-packages/google/protobuf/runtime_version.py:98: UserWarning: Protobuf gencode version 5.29.3 is exactly one major version older than the runtime version 6.31.1 at a2a.proto. Please update the gencode to avoid compatibility violations in the next runtime release.
warnings.warn(
Traceback (most recent call last):
File "", line 198, in _run_module_as_main
File "", line 88, in _run_code
File "/path/to/a2a-samples/samples/python/agents/travel_planner_agent/__main__.py", line 6, in
from a2a.types import (
...<4 lines>...
)
ImportError: cannot import name 'AgentAuthentication' from 'a2a.types' (/path/to/a2a-samples/samples/python/agents/travel_planner_agent/.venv/lib/python3.13/site-packages/a2a/types.py)
```

### Code of Conduct

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

O erro ocorre em __main__.py na linha 6, onde ele tenta importar 'AgentAuthentication' de 'a2a.types'. Verifique o módulo a2a.types no pacote instalado para ver quais exportações estão disponíveis. Compare as dependências de travel_planner_agent com a versão da biblioteca a2a. A correção provavelmente envolve atualizar a instrução de importação ou a versão da biblioteca.

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