API reference now shows the response bodies the endpoints return

The reference for the customer, order, invoice, payment, dispute, credit and cost endpoints described a response body that no endpoint has ever returned. It is now written from the responses themselves.

• Field names match the response, so a generated client reads a real value instead of undefined
• Money and quantity fields are documented as decimal strings, which is how they arrive, rather than as numbers
• List endpoints are documented with their data and pagination envelope instead of a bare array
• Deleting a customer, contact, invoice or credit bundle is documented as a 200 with a confirmation message, which is the response you get
• Adding an order line is documented as the single POST it is, rather than a bulk update that was never available
• The in-app documentation examples for these objects were rewritten to match, so the two no longer disagree
• A contract test keeps these schemas tied to the endpoints, so the reference cannot drift away again unnoticed