[Bug]: Add python sdk as proper dependency

[juliangrueber]

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.

a2a-tck/python-sut/tck_core_agent/pyproject.toml

Line 16 in 50afdd6

a2a-sdk = { path="../../../a2a-python" }

Wondering whether it makes to

1. Convert the source to pypip
2. Adding the clone as prerequisite to the ReadMe

Relevant log output

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

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