Merge pull request #20984 from notomo/fix-message-kind-on-history

fix(ui-ext): correct message kind in history before vim.ui_attach()

src/nvim/message.c

@@ -3153,6 +3153,7 @@ int msg_end(void)
 void msg_ext_ui_flush(void)
 {
   if (!ui_has(kUIMessages)) {
+    msg_ext_kind = NULL;
     return;
   }