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

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

The issue is about a version mismatch between the a2a-sdk (0.2.12) and the langgraph sample code written for 0.2.8. First, locate the sample code in the repository, likely in a directory like 'langgraph' or 'samples'. Examine the import statements that are failing. Then, check the a2a-sdk changelog or source to see what changed between versions. The fix likely involves updating the sample code's imports or dependencies to be compatible with 0.2.12. 'Done' means the sample runs without import errors.

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