fix: assignee_changed callback not getting triggered during conversation creation (#9334)

The reload method in our callback was refreshing the object and hence the saved_change_to_assignee_id? Method wasn't working in the following callbacks.

This impacted the listeners subscribing to the event `ASSIGNEE_CHANGE`, `TEAM_CHANGE` etc

spec/models/concerns/auto_assignment_handler_shared.rb

@@ -26,10 +26,6 @@ shared_examples_for 'auto_assignment_handler' do
       expect(conversation.reload.assignee).to eq(agent)
     end
 
-    it 'adds assignee to conversation participants' do
-      expect(conversation.conversation_participants.map(&:user)).to include(agent)
-    end
-
     it 'will not auto assign agent if enable_auto_assignment is false' do
       inbox.update(enable_auto_assignment: false)