- listHelcimCustomerTransactions(customerCode): GET /card-transactions/
with customerCode filter, sorts newest-first, caps at 50, normalizes
Helcim status (APPROVED/DECLINED) + type (refund) into
paid/refunded/failed/other.
- updateHelcimCustomerDefaultPaymentMethod(customerId, cardToken):
resolves cardToken -> cardId via /customers/{id}/cards, then PATCHes
/customers/{id}/cards/{cardId}/default.
- updateHelcimSubscriptionPaymentMethod(subscriptionId, cardToken):
wraps updateHelcimSubscription with a cardToken payload.
- helcimUpdateCardSchema: Zod schema { cardToken: string } for the
upcoming /api/helcim/update-card route.
- Unit tests for all three helpers (success + error paths).
