feat(payment): expand Stripe methods and checkout handoff

src/lib/payment/payment_launch.ts

@@ -143,6 +143,27 @@ export function getStripeClientSecret(
   return null;
 }
 
+export function getStripeCustomerSessionClientSecret(
+  payParams: Record<string, unknown>,
+): string | null {
+  const provider = payParams.provider;
+  const isStripeProvider =
+    typeof provider !== "string" || provider.toLowerCase() === "stripe";
+  const enabled = payParams.savedPaymentMethodsEnabled === true;
+  const clientSecret = payParams.customerSessionClientSecret;
+
+  if (
+    isStripeProvider &&
+    enabled &&
+    typeof clientSecret === "string" &&
+    /^cuss_[^\s]+_secret_[^\s]+$/i.test(clientSecret)
+  ) {
+    return clientSecret;
+  }
+
+  return null;
+}
+
 export interface LaunchEzpayRedirectInput {
   orderId: string | null;
   paymentUrl: string;