Missing dependency and outdated README

Branch: main
README.md for the github-agent sample `samples/python/agents/github-agent` is outdated:
```
uv run . http://localhost:10007
> Using CPython 3.13.9
> Creating virtual environment at: ...
> Built a2a-sample-client-cli ...
> Usage: . [OPTIONS]
> Try '. --help' for help. > Error: Got unexpected extra argument (http://localhost:10007)
```

Missing dependency: fastapi
```
class A2AFastAPI(FastAPI):
raise TypeError(f"Cannot subclass {self!r}") TypeError: Cannot subclass typing.Any
in .venv\lib\site-packages\a2a\server\apps\jsonrpc\fastapi_app.py", line 28
```

Culprit
```
try:
from fastapi import FastAPI
_package_fastapi_installed = True
except ImportError:
FastAPI = Any # missing dependency is harder to debug this way!
_package_fastapi_installed = False
```

The issue is in the github-agent sample at `samples/python/agents/github-agent`. First, check the README.md for outdated instructions. Then, examine the dependency handling in `.venv/lib/site-packages/a2a/server/apps/jsonrpc/fastapi_app.py` around line 28 where FastAPI is conditionally imported. Add 'fastapi' to the project's dependencies (likely in pyproject.toml or requirements.txt). Test by running the command `uv run . http://localhost:10007` to verify the fix.

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