Missing fastapi dependency on a2a_gui

### What happened?

After running:
```
uv run uvicorn server:app --host 0.0.0.0 --port 8000
```
I get this error
```
ModuleNotFoundError: No module named 'fastapi'
```

### Relevant log output

```shell
from fastapi import FastAPI, Request
ModuleNotFoundError: No module named 'fastapi'
```

### Code of Conduct

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

Check the project's dependency file (likely requirements.txt or pyproject.toml) to see if fastapi is listed. The error occurs when starting the server with uvicorn. Add fastapi to the dependencies, then test by running the uvicorn command again. The fix is done when the server starts without the ModuleNotFoundError.

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