Add integration test for notification delivery and simplify test helpers

- Add comprehensive integration test covering card assignment, comments,
  mentions, email bundling, and edge cases (system user, inactive user)
- Inline push notification test helpers into the only test that uses them
- Remove separate PushNotificationTestHelper module

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

test/models/notification/push_target/web_test.rb

@@ -77,5 +77,4 @@ class Notification::PushTarget::WebTest < ActiveSupport::TestCase
 
     Notification::PushTarget::Web.new(notification).process
   end
-
 end