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