Function spawn_forward_task 

pub fn spawn_forward_task(
    buffer_rx: UnboundedReceiver<StreamChunk>,
    tx: Sender<StreamChunk>,
    cancel: CancellationToken,
) -> JoinHandle<()>

启动 Forward Task：从 BufferedSink 读取，转发到 mpsc channel。

消费者断开（Receiver dropped）时，task 退出并触发 CancellationToken。