I need to test my payment flows without hitting the real Stripe (or my payment provider's) API, and without depending on their test mode being reachable. First, find every place in my codebase that calls the payment provider's SDK or API: checkout session creation, webhook handling, subscription updates, refunds, customer creation. List them for me. Then build a mock layer for tests that: 1. Intercepts calls to the provider SDK and returns realistic responses shaped like the real API's responses (including the fields my code actually reads). 2. Can simulate the important scenarios: successful payment, declined card, webhook with a valid signature, webhook with an invalid signature, duplicate webhook delivery, subscription created, subscription canceled, payment failed on renewal. 3. Lets individual tests choose which scenario to trigger without rewriting the mock. Write tests using the mock for my existing payment logic: checkout completion, webhook handling, and subscription state changes. Assert what my database looks like after each scenario, not just that the mock was called. Don't change my production payment code to accommodate the mock unless the current code is genuinely untestable. If it is, show me why first. When done, run the tests and show me the results, plus a short note on which real-world payment scenarios are still untested.