Function run_collect 

pub async fn run_collect<I, O, F, Fut>(
    rx: UnboundedReceiver<I>,
    parallelism: usize,
    stage: F,
) -> Vec<O>
where
    I: Send + 'static,
    O: Send + 'static,
    F: FnMut(I) -> Fut + Send + 'static,
    Fut: Future<Output = O> + Send + 'static,

Run a single-stage pipeline end-to-end: pull from rx, apply the async stage with the given parallelism, and collect every output into a Vec. The future completes when every sender is dropped upstream.