DEV: adds a :chat_thread_created trigger (#24133)

Usage:

```ruby
DiscourseEvent.on(:chat_thread_created) do |thread|
end
```

plugins/chat/app/services/chat/create_thread.rb

@@ -26,6 +26,7 @@ module Chat
       step :associate_thread_to_message
       step :fetch_membership
       step :publish_new_thread
+      step :trigger_chat_thread_created_event
     end
 
     # @!visibility private
@@ -84,5 +85,9 @@ module Chat
     def publish_new_thread(channel:, original_message:, **)
       ::Chat::Publisher.publish_thread_created!(channel, original_message, context.thread.id)
     end
+
+    def trigger_chat_thread_created_event
+      ::DiscourseEvent.trigger(:chat_thread_created, context.thread)
+    end
   end
 end

plugins/chat/spec/services/chat/create_thread_spec.rb

@@ -51,6 +51,12 @@ RSpec.describe Chat::CreateThread do
         expect(message.data["type"]).to eq("thread_created")
       end
 
+      it "triggers a discourse event `chat_thread_created`" do
+        event = DiscourseEvent.track_events(:chat_thread_created) { result }.first
+
+        expect(event[:params][0]).to eq(result.thread)
+      end
+
       it "sets the title when existing" do
         params[:title] = "Restaurant for Saturday"
         result