a2a-samples python langgraph version error

### What happened?

When I run pip install a2a-sdk, version 0.2.12 gets installed by default.
However, the provided example code(a2a langgraph sample) seems to be based on version 0.2.8.
With 0.2.12, I get import errors right from the first lines of the example code.

please fix it

### Relevant log output

```shell

```

### Code of Conduct

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