End a running activity, referenced by its instance_id. On iOS, the system dismisses the activity from the Lock Screen. On Android, the SDK normally renders the final content as a non-ongoing notification; it cancels the notification only when there's no final notification to render. You can include a final content_state to show before the activity ends.

push_payload is optional on the end call, whether Customer.io delivers directly through APNs or relays through Firebase Cloud Messaging (FCM). If you include a push_payload.alert, it must contain both a title and a body.