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/topic_admin_menu_spec.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+describe "Topic Admin Menu", type: :system do
+  fab!(:admin)
+  let(:topic_page) { PageObjects::Pages::Topic.new }
+
+  before { sign_in(admin) }
+
+  context "for a PM" do
+    fab!(:pm) { Fabricate(:private_message_topic, title: "Can you help me with this?") }
+    fab!(:op) { Fabricate(:post, topic: pm, user: admin, created_at: 1.day.ago) }
+    fab!(:category)
+
+    it "shows the errors when converting to a public topic is not possible" do
+      # create a topic with the same title to force a "duplicate title" error
+      Fabricate(:topic, title: pm.title, category: category)
+
+      topic_page.visit_topic(pm)
+      topic_page.move_to_public_category(category)
+      expect(topic_page.move_to_public_modal).to have_css("#modal-alert")
+    end
+  end
+end