Declined Cards in C#

Not every payment goes through: the card has no money left, it is blocked, or the bank says no. Then the payment provider answers 402 Payment Required with an error instead of a payment. Our provider declines every amount over 1000 and answers {"error": "card_declined"}. There is no "id" in it, so payment.GetProperty("id") throws and the customer gets a 500.

Check the status code first. Before you use another service's answer, look at response.StatusCode. The same idea with the supplier and a product it doesn't sell:

var response = await http.GetAsync(SUPPLIER + "/stock/99");
if ((int)response.StatusCode == 404) {
    return Results.Json(new { error = "Unknown product" }, statusCode: 404);
}
var data = await response.Content.ReadFromJsonAsync<JsonElement>();

A declined card is not a bug in your shop, so the answer is not 500. It is not 502 either: the provider worked and gave a clear answer. Answer 402 yourself, pass the provider's reason on, and return before the UPDATE, so the order stays pending and the customer can try another card.

POST /orders/9/pay  →  402  {"error": "Payment declined", "reason": "card_declined"}
GET /orders/9       →  200  {…, "payment_id": null, "status": "pending", "total": 1250}
POST /orders/8/pay  →  201  {"amount": 80, "order_id": 8, "payment_id": "pay_1"}

Your task: in the /orders/{id}/pay route, right after the line that reads payment, check whether the provider answered 402. If it did, answer "Payment declined" with 402, and pass the provider's "error" on under reason.

Assignment
Follow the Coding Tutorial to handle a declined card in POST /orders/{id}/pay.

Hint
The provider's reason is payment.GetProperty("error"). A return ends the lambda, so with the if above the UPDATE command, a declined order is never marked paid.