DEV: create new_post_moved event trigger when moving posts (#24005)

This change adds a new event trigger (new_post_moved) when the first post in a topic is moved to a new topic.

Plugins that listen for the new_post_moved event now have an easy way to update old data based on the post id.

app/models/post_mover.rb

@@ -277,6 +277,7 @@ class PostMover
     new_post.custom_fields = post.custom_fields
     new_post.save_custom_fields
 
+    DiscourseEvent.trigger(:first_post_moved, new_post, post)
     DiscourseEvent.trigger(:post_moved, new_post, original_topic.id)
 
     # we don't want to keep the old topic's OP bookmarked when we are

spec/models/post_mover_spec.rb

@@ -2463,5 +2463,45 @@ RSpec.describe PostMover do
         end
       end
     end
+
+    context "with event trigger" do
+      fab!(:topic_1) { Fabricate(:topic) }
+      fab!(:topic_2) { Fabricate(:topic) }
+      fab!(:post_1) { Fabricate(:post, topic: topic_1) }
+      fab!(:post_2) { Fabricate(:post, topic: topic_1) }
+
+      it "receives 2 post moved event triggers for the first post" do
+        post_mover = PostMover.new(topic_1, Discourse.system_user, [post_1.id])
+        events = DiscourseEvent.track_events { post_mover.to_topic(topic_2.id) }
+        filtered_events =
+          events.filter { |e| %i[first_post_moved post_moved].include? e[:event_name] }
+
+        expect(filtered_events.size).to eq(2)
+      end
+
+      it "uses first_post_moved trigger for first post" do
+        post_mover = PostMover.new(topic_1, Discourse.system_user, [post_1.id])
+        events = DiscourseEvent.track_events(:first_post_moved) { post_mover.to_topic(topic_2.id) }
+        expect(events.size).to eq(1)
+
+        new_post = Post.find_by(topic_id: topic_2.id, post_number: 1)
+
+        event = events.first
+        expect(event[:event_name]).to eq(:first_post_moved)
+        expect(event[:params][0]).to eq(new_post)
+        expect(event[:params][1]).to eq(post_1)
+      end
+
+      it "uses post_moved trigger for other posts" do
+        post_mover = PostMover.new(topic_1, Discourse.system_user, [post_2.id])
+        events = DiscourseEvent.track_events(:post_moved) { post_mover.to_topic(topic_2.id) }
+        expect(events.size).to eq(1)
+
+        event = events.first
+        expect(event[:event_name]).to eq(:post_moved)
+        expect(event[:params][0]).to eq(post_2)
+        expect(event[:params][1]).to eq(topic_1.id)
+      end
+    end
   end
 end