restructured tests and fix type errors

server/tests/integration/billing/update-subscription/update-quantity/decrease-quantity.test.ts

@@ -0,0 +1,85 @@
+import { expect, test } from "bun:test";
+import type { ApiCustomerV3 } from "@autumn/shared";
+import { expectLatestInvoiceCorrect } from "@tests/integration/billing/utils/expectLatestInvoiceCorrect.js";
+import { TestFeature } from "@tests/setup/v2Features.js";
+import { items } from "@tests/utils/fixtures/items.js";
+import { products } from "@tests/utils/fixtures/products.js";
+import { initScenario, s } from "@tests/utils/testInitUtils/initScenario.js";
+import chalk from "chalk";
+
+/**
+ * Subscription Update - Decrease Quantity Tests
+ *
+ * These tests verify quantity downgrades:
+ * - Basic single-feature downgrade (20 → 5 units)
+ */
+
+test.concurrent(
+	`${chalk.yellowBright("update-quantity: downgrade 20 to 5 units")}`,
+	async () => {
+		const customerId = "dec-qty-basic-downgrade";
+		const billingUnits = 12;
+		const pricePerUnit = 8;
+
+		const prepaidItem = items.prepaid({
+			featureId: TestFeature.Messages,
+			billingUnits,
+			price: pricePerUnit,
+		});
+
+		const product = products.base({
+			id: "prepaid",
+			items: [prepaidItem],
+		});
+
+		const { autumnV1 } = await initScenario({
+			customerId,
+			setup: [
+				s.customer({ paymentMethod: "success" }),
+				s.products({ list: [product] }),
+			],
+			actions: [
+				s.attach({
+					productId: product.id,
+					options: [
+						{ feature_id: TestFeature.Messages, quantity: 20 * billingUnits },
+					],
+				}),
+			],
+		});
+
+		// Preview the downgrade
+		const preview = await autumnV1.subscriptions.previewUpdate({
+			customer_id: customerId,
+			product_id: product.id,
+			options: [
+				{ feature_id: TestFeature.Messages, quantity: 5 * billingUnits },
+			],
+		});
+
+		// Verify preview total matches expected (20 -> 5 = -15 units * $8)
+		expect(preview.total).toBe(-15 * pricePerUnit);
+
+		// Downgrade from 20 to 5 units
+		await autumnV1.subscriptions.update({
+			customer_id: customerId,
+			product_id: product.id,
+			options: [
+				{ feature_id: TestFeature.Messages, quantity: 5 * billingUnits },
+			],
+		});
+
+		const customer = await autumnV1.customers.get<ApiCustomerV3>(customerId);
+		const feature = customer.features?.[TestFeature.Messages];
+
+		// Should have 60 messages (5 units × 12 billing_units)
+		expect(feature?.balance).toBe(60);
+
+		// Expect credit invoice for downgrade (20 -> 5 = -15 units * $8)
+		expectLatestInvoiceCorrect({
+			customer,
+			productId: product.id,
+			amount: -15 * pricePerUnit,
+		});
+	},
+);