FIX: Unread topics not clearing when whisper is last post (#8271)

Meta thread: https://meta.discourse.org/t/cant-dismiss-unread-if-last-post-is-an-assign-or-whisper/131823/7

* when sending a whisper, the highest_staff_post_number is set
in the next_post_number method for a Topic, but the
highest_post_number is left alone. this leaves a situation
where highest_staff_post_number is > highest_post_number
* when TopicsBulkAction#dismiss_posts was run, it was only setting the topic_user
highest_seen_post_number using the highest_post_number from the topic, so if
the user was staff and the last post in a topic was a whisper
their highest seen number was not set, and the topic stayed unread

Found through testing that the bug wasn't to do with Assign/Unassign as they do not affect the post numbers, only whispering does.

spec/components/topics_bulk_action_spec.rb

@@ -20,6 +20,32 @@ describe TopicsBulkAction do
       expect(tu.last_read_post_number).to eq(3)
       expect(tu.highest_seen_post_number).to eq(3)
     end
+
+    context "when the user is staff" do
+      fab!(:user) { Fabricate(:admin) }
+
+      context "when the highest_staff_post_number is > highest_post_number for a topic (e.g. whisper is last post)" do
+        it "dismisses posts" do
+          post1 = create_post(user: user)
+          p = create_post(topic_id: post1.topic_id)
+          create_post(topic_id: post1.topic_id)
+
+          whisper = PostCreator.new(
+            user,
+            topic_id: post1.topic.id,
+            post_type: Post.types[:whisper],
+            raw: 'this is a whispered reply'
+          ).create
+
+          TopicsBulkAction.new(user, [post1.topic_id], type: 'dismiss_posts').perform!
+
+          tu = TopicUser.find_by(user_id: user.id, topic_id: post1.topic_id)
+
+          expect(tu.last_read_post_number).to eq(4)
+          expect(tu.highest_seen_post_number).to eq(4)
+        end
+      end
+    end
   end
 
   describe "invalid operation" do