feat(order): name the fields an order still lacks for a checkout

missingCheckoutFields returns which of email, invoiceAddress and
delivery an order lacks, so the checkout can answer 400 before it
creates a PayPal order.

Refs libretech/mp#71

tests/unit/order-state.test.ts

@@ -1,6 +1,6 @@
 import { test } from "node:test";
 import assert from "node:assert/strict";
-import { isLockedForCustomer } from "../../src/checkout/order-state.ts";
+import { isLockedForCustomer, missingCheckoutFields } from "../../src/checkout/order-state.ts";
 
 // Shaped like the row PUT /orders/:uuid/cart reads with strapi.db.query("api::order.order").findOne before it updates.
 const order = (overrides = {}) => ({
@@ -21,6 +21,36 @@ test("an order is open for the customer until its payment is authorised", () =>
 });
 
 test("an order is locked once its payment is authorised", () => {
-	// what capturePayment writes once PayPal reports the capture COMPLETED
+	// what capturePayment writes once the capture is verified (src/checkout/capture-payment.ts)
+	assert.equal(isLockedForCustomer(order({ paymentAuthorised: true, paymentStatus: "captured" })), true);
+	// what it wrote before
 	assert.equal(isLockedForCustomer(order({ paymentAuthorised: true, paymentStatus: "authorised" })), true);
 });
+
+// Shaped like the row POST /orders/:uuid/checkout reads (strapi.db.query findOne with orderDefaultParams), after both checkout steps.
+const readyForCheckout = (overrides = {}) =>
+	order({
+		invoiceAddress: "Erika Mustermann\nMusterstraße 1\n70190 Stuttgart",
+		deliveryAddress: "Erika Mustermann\nMusterstraße 1\n70190 Stuttgart",
+		delivery: { id: 1, name: "DHL", price: 4.95 },
+		...overrides
+	});
+
+test("an order that went through both checkout steps lacks nothing for the checkout", () => {
+	assert.deepEqual(missingCheckoutFields(readyForCheckout()), []);
+});
+
+test("names the email, invoice address and delivery method an order still lacks for the checkout", () => {
+	// a new order: POST /orders creates it empty
+	assert.deepEqual(missingCheckoutFields({ email: null, invoiceAddress: null, delivery: null }), ["email", "invoiceAddress", "delivery"]);
+	assert.deepEqual(missingCheckoutFields({}), ["email", "invoiceAddress", "delivery"]);
+	// after step 1 alone
+	assert.deepEqual(missingCheckoutFields(readyForCheckout({ invoiceAddress: null, deliveryAddress: null, delivery: null })), [
+		"invoiceAddress",
+		"delivery"
+	]);
+	assert.deepEqual(missingCheckoutFields(readyForCheckout({ email: "  " })), ["email"]);
+	assert.deepEqual(missingCheckoutFields(readyForCheckout({ invoiceAddress: "" })), ["invoiceAddress"]);
+	// a delivery method that was deleted is populated as null
+	assert.deepEqual(missingCheckoutFields(readyForCheckout({ delivery: null })), ["delivery"]);
+});