[Bug]: Add python sdk as proper dependency

### What happened?

I was trying to run the example system under test, however, I got a an error as the dependency referring to a local directory.

https://github.com/a2aproject/a2a-tck/blob/50afdd61169b3a6e099df5147d3282b0acf91307/python-sut/tck_core_agent/pyproject.toml#L16

Wondering whether it makes to
1. Convert the source to pypip
2. Adding the clone as prerequisite to the ReadMe

### Relevant log output

```shell
`
error: Distribution not found at: file:///local/home/****/a2a-python
`
```

### Code of Conduct

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

The issue points to a line in python-sut/tck_core_agent/pyproject.toml where a local file dependency is used. First, check if the python SDK is published on PyPI or if it needs to be packaged. Update the dependency to a proper PyPI reference or add setup instructions to clone the SDK locally. Verify by running the example system after the change.

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