async fn send_pending_messages(
writer: &mut SplitSink<WebSocket, Message>,
pending_messages: &mut EnvelopeBatch,
) -> Result<usize, WebSocketCloseCode>async fn send_pending_messages(
writer: &mut SplitSink<WebSocket, Message>,
pending_messages: &mut EnvelopeBatch,
) -> Result<usize, WebSocketCloseCode>