FIX: shows errors when converting to public is impossible (#27103)

Follow up to e04ac5e2d8 which missed showing the error messages in the "convert to public" modal 🤦‍♂️

spec/system/page_objects/pages/topic.rb

@@ -228,6 +228,18 @@ module PageObjects
         post_by_number(post).has_css?(".read-state.read", visible: :all, wait: 3)
       end
 
+      def move_to_public_category(category)
+        click_admin_menu_button
+        find(".topic-admin-menu-content li.topic-admin-convert").click
+        move_to_public_modal.find(".category-chooser").click
+        find(".category-row[data-value=\"#{category.id}\"]").click
+        move_to_public_modal.find(".btn-primary").click
+      end
+
+      def move_to_public_modal
+        find(".modal.convert-to-public-topic")
+      end
+
       private
 
       def within_post(post)