Function start_reaper 

pub fn start_reaper(
    manager: Arc<SessionManager>,
    cancel: CancellationToken,
) -> JoinHandle<()>

Spawn a background task that periodically reaps idle sessions.

The reaper runs every 30 seconds, checking for sessions whose last activity exceeds their configured idle timeout. Returns immediately — the task runs until the CancellationToken is canceled (or the process exits).