Function spawn_fetch 

fn spawn_fetch<S, F, Fut, T>(
    clients: &HashMap<String, Arc<dyn SashikoApi>, S>,
    active_remote: &str,
    msg_tx: &UnboundedSender<Message>,
    fetch: F,
    on_missing: fn(&str) -> Message,
)
where
    S: BuildHasher,
    F: FnOnce(Arc<dyn SashikoApi>) -> Fut + Send + 'static,
    Fut: Future<Output = T> + Send,
    T: Into<Message> + Send + 'static,

Spawn an async fetch task if the client exists, or send an error message.

This helper eliminates the repeated if let Some(client) = clients.get(...) { clone + spawn } else { send error } pattern across all fetch commands.