feat(events): harden registration checkout lifecycle

apps/payments/tests/integration/test_payment_admin.py

@@ -86,7 +86,8 @@ def test_payment_callback_verification_failures_notify_once(client, user_factory
     else:
         gateway.return_value.json.return_value = gateway_result
     response = client.get("/api/payments/callback", {"Authority": "AUTH-FAIL", "Status": "OK"})
-    assert response.status_code == 302 and "status=pending" in response.url
+    assert response.status_code == 302
+    assert f"/events/{event.slug}/registration?registration_id={registration.ticket_id}" in response.url
     payment.refresh_from_db()
     registration.refresh_from_db()
     assert payment.status == Payment.OrderStatusChoices.PENDING