Add auto_shutdown => any_significant to rabbit_fifo_dlx_worker_sup

When the DLX worker exits gracefully (e.g., `{shutdown, queue_leader_down}`
on leadership change), a `transient` child is not restarted and does not
trip the intensity counter. Without `auto_shutdown`, the worker_sup
survives as an empty orphan under the sup_sup.

Marking the worker as `significant => true` and the supervisor as
`auto_shutdown => any_significant` ensures the worker_sup self-terminates
when its worker exits gracefully. Same pattern as `rabbit_channel_sup`.

Author: lukebakken

deps/rabbit/src/rabbit_fifo_dlx_worker_sup.erl

@@ -21,10 +21,12 @@ init([QRef]) ->
     DlxWorkerSupPid = self(),
     SupFlags = #{strategy => one_for_one,
                  intensity => 0,
-                 period => 1},
+                 period => 1,
+                 auto_shutdown => any_significant},
     ChildSpec = #{id => rabbit_fifo_dlx_worker,
                   start => {rabbit_fifo_dlx_worker, start_link, [QRef, DlxWorkerSupPid]},
                   type => worker,
                   restart => transient,
+                  significant => true,
                   shutdown => 5000},
     {ok, {SupFlags, [ChildSpec]}}.