chore: API improvements (#3637)

- Unique validations for Inbox members and Team member objects
- Move notification processing to Async

spec/models/concerns/assignment_handler_shared.rb

@@ -78,19 +78,9 @@ shared_examples_for 'assignment_handler' do
       expect(conversation.reload.assignee).to eq(agent)
     end
 
-    it 'creates a new notification for the agent' do
+    it 'dispaches assignee changed event' do
+      expect(EventDispatcherJob).to(have_been_enqueued.at_least(:once).with('assignee.changed', anything, anything))
       expect(update_assignee).to eq(true)
-      expect(agent.notifications.count).to eq(1)
-    end
-
-    it 'does not create assignment notification if notification setting is turned off' do
-      notification_setting = agent.notification_settings.first
-      notification_setting.unselect_all_email_flags
-      notification_setting.unselect_all_push_flags
-      notification_setting.save!
-
-      expect(update_assignee).to eq(true)
-      expect(agent.notifications.count).to eq(0)
     end
 
     context 'when agent is current user' do