cyper-axum: `Sse::keep_alive()` will panic

```
thread panicked at axum-0.8.9/src/response/sse.rs:591:26:
there is no reactor running, must be called from the context of a Tokio 1.x runtime
```

because it uses `tokio::time::Sleep`

Start by reproducing the panic from Sse::keep_alive() and inspect the referenced axum-0.8.9/src/response/sse.rs location, especially its use of tokio::time::Sleep. Done means keep-alive no longer panics when used with cyper's runtime; add or run a regression test if the repository provides a relevant test entry point.

Written by the indexing model from the issue text.