FIX: Notify tag watchers when publishing topic (#17576)

When a topic was published from a shared draft and it had tags, the
users watching the tags were not notified. The problem was that the
topics are usually created in a secret category and publishing it just
moves an existent topic to the target category, without making any
changes to the tags.

spec/lib/topic_publisher_spec.rb

@@ -16,6 +16,8 @@ describe TopicPublisher do
       fab!(:shared_draft) { Fabricate(:shared_draft, topic: topic, category: category) }
       fab!(:moderator) { Fabricate(:moderator) }
       fab!(:op) { Fabricate(:post, topic: topic) }
+      fab!(:user) { Fabricate(:user) }
+      fab!(:tag) { Fabricate(:tag) }
 
       before do
         # Create a revision
@@ -51,6 +53,24 @@ describe TopicPublisher do
           expect(op.last_version_at).to eq_time(published_at)
         end
       end
+
+      it "will notify users watching tag" do
+        Jobs.run_immediately!
+
+        TagUser.create!(
+          user_id: user.id,
+          tag_id: tag.id,
+          notification_level: NotificationLevels.topic_levels[:watching]
+        )
+
+        topic.update!(tags: [tag])
+
+        expect { TopicPublisher.new(topic, moderator, shared_draft.category_id).publish! }
+          .to change { Notification.count }.by(1)
+
+        topic.reload
+        expect(topic.tags).to contain_exactly(tag)
+      end
     end
 
   end