Build-an-X for push notification tests (push notifications, web push, FCM / APNs push messages) across Web Push (RFC 8030 / VAPID), Apple Push Notification Service (APNs), and Firebase Cloud Messaging (FCM) - covers subscription handshake, payload encryption, badge / sound / click-action assertions, expired-subscription cleanup, silent-vs-alert, and topic-vs-targeted routing. Use when authoring tests for any push notification flow.