Function send_to_session_socket 

pub async fn send_to_session_socket(
    socket_path: &str,
    payload: &[u8],
) -> Result<()>

Deliver one payload to a session endpoint: connect, write, close. The listener reads to EOF, and a named pipe has no half-close (shutdown only flushes), so EOF reaches it only when the handle is dropped. Doing all three here means no caller can hold the pipe open by accident.