Put two payment SDKs behind one gateway.

A shop app takes card payments through two vendors, and the checkout talks to both SDKs directly: it converts rupees to paise for one, reads a status string from it, reads an integer code from the other, and branches on the provider's name in pay and refund. A third vendor, or a test, means editing the checkout again. The vendors' SDKs are given and cannot be changed. Move the checkout behind a PaymentGateway interface and write one adapter per vendor.

The behaviour to keep, exactly:

Build this API (names are Python's; JavaScript and TypeScript use the same names, with refund, charge and pay as written):

• PaymentDeclined: the exception for a decline (an Error subclass in JavaScript and TypeScript).
• AlphaAdapter(sdk) and BetaAdapter(sdk): each has charge(customer, amount, idempotency_key), which returns the payment reference (a string) or raises PaymentDeclined, and refund(payment_ref, amount). amount is always in rupees.
• Checkout(gateway) with pay(order_id, customer, amount) and refund(order_id, amount). It remembers the reference of each paid order, and it uses the order id as the idempotency key. It catches PaymentDeclined and nothing else, and it never names a vendor.

The tests check every old number, that each SDK sees its own unit, that a fake gateway written inside a test works, that a third vendor written inside a test plugs in without any change to Checkout, and that no provider name is left in the code.