Automated tests for a payment service API

Our team is looking for a test automation engineer to cover the public API of our payment service with automated checks. The service handles card and wallet payments, so we need far more than the happy path: dozens of failure cases must be verified, including insufficient funds, expired cards, bank declines, provider timeouts, and repeated requests. Idempotency matters most of all — the same key must never charge a customer twice, and a retried request has to return the original result. We expect scenarios for creating a payment, confirming it, cancelling, and issuing partial or full refunds, plus verification of webhooks and status transitions. The suite must run inside our build pipeline automatically, fail on any regression, and produce a readable report with coverage and a list of failed assertions. Experience with sandbox environments and mocked bank responses is a strong plus. Please tell us which stack you propose, how you plan to manage test data, and how long the first working run will take.