Function serve_one_admin_socket 

pub fn serve_one_admin_socket(
    socket_path: &str,
    snapshot: &AdminSnapshot,
) -> Result<AdminReply, AdminConnectionError>

Run one blocking local-socket accept and serve exactly one admin request.

This is the admin-side counterpart to serve_one_local_socket for Hello. The full long-lived broker loop can reuse handle_admin_connection after selecting an admin connection from the shared accept path.