fix(exit): close memfiles after processing events (#30872)

Problem:  When exiting, processed events may still use memfiles after
          they are closed.
Solution: Close memfiles after processing events.

src/nvim/main.c

@@ -678,12 +678,14 @@ void os_exit(int r)
   } else {
     ui_flush();
     ui_call_stop();
-    ml_close_all(true);           // remove all memfiles
   }
 
   if (!event_teardown() && r == 0) {
     r = 1;  // Exit with error if main_loop did not teardown gracefully.
   }
+  if (!ui_client_channel_id) {
+    ml_close_all(true);  // remove all memfiles
+  }
   if (used_stdin) {
     stream_set_blocking(STDIN_FILENO, true);  // normalize stream (#2598)
   }