fix(exit): the TUI should not ui_flush() itself (#21625)

src/nvim/main.c

@@ -627,7 +627,9 @@ void os_exit(int r)
 {
   exiting = true;
 
-  ui_flush();
+  if (!ui_client_channel_id) {
+    ui_flush();
+  }
   ui_call_stop();
   ml_close_all(true);           // remove all memfiles
 

src/nvim/ui.c

@@ -506,6 +506,7 @@ handle_T ui_cursor_grid(void)
 
 void ui_flush(void)
 {
+  assert(!ui_client_channel_id);
   if (!ui_active()) {
     return;
   }