feat(payment): expand Stripe methods and checkout handoff

src/lib/payment/__tests__/payment_launch.test.ts

@@ -2,6 +2,7 @@ import { describe, expect, it, vi } from "vitest";
 
 import {
   getPaymentUrl,
+  getStripeCustomerSessionClientSecret,
   getStripeClientSecret,
   isEzpayPayment,
   launchEzpayRedirect,
@@ -43,6 +44,28 @@ describe("payment launch helpers", () => {
     ).toBeNull();
   });
 
+  it("enables saved cards only with an explicit flag and a Customer Session secret", () => {
+    expect(
+      getStripeCustomerSessionClientSecret({
+        provider: "stripe",
+        customerSessionClientSecret: "cuss_123_secret_456",
+        savedPaymentMethodsEnabled: true,
+      }),
+    ).toBe("cuss_123_secret_456");
+    expect(
+      getStripeCustomerSessionClientSecret({
+        customerSessionClientSecret: "cuss_123_secret_456",
+        savedPaymentMethodsEnabled: false,
+      }),
+    ).toBeNull();
+    expect(
+      getStripeCustomerSessionClientSecret({
+        customerSessionClientSecret: "invalid-customer-session",
+        savedPaymentMethodsEnabled: true,
+      }),
+    ).toBeNull();
+  });
+
   it("resolves Ezpay URL and QR launch parameters without treating QR data as a URL", () => {
     expect(
       resolveEzpayLaunchTarget({