Function spawn_background 

pub async fn spawn_background<F, Fut>(
    thread_mgr: SharedThreadManager,
    options: SpawnOptions,
    task_fn: F,
) -> SubtaskHandle<()>
where
    F: FnOnce(CancellationToken, SharedThreadManager) -> Fut + Send + 'static,
    Fut: Future<Output = Result<(), String>> + Send + 'static,

Spawn a long-running background thread.

Unlike subagents and tasks, background threads don’t have a natural return value. They run until cancelled.