fix(payments-next): Upgrading subscription set to cancel does not unset cancelled_for_customer_at

Because:

* When upgrading a subscription that has previously been set to cancel but has not cancelled yet, the metadata field cancelled_for_customer_at is not unset even though we uncancel the subscription as part of the upgrade process.

This commit:

* Removes cancelled_for_customer_at on subscription upgrade.

Closes #[PAY-3586](https://mozilla-hub.atlassian.net/browse/PAY-3586)

Author: elizabeth-ilina

libs/payments/cart/src/lib/checkout.service.spec.ts

@@ -2234,6 +2234,16 @@ describe('CheckoutService', () => {
         mockAttributionData
       );
       expect(subscriptionManager.update).toHaveBeenCalledTimes(2);
+      expect(subscriptionManager.update).toHaveBeenNthCalledWith(
+        1,
+        subscription.id,
+        expect.objectContaining({
+          cancel_at_period_end: false,
+          metadata: expect.objectContaining({
+            cancelled_for_customer_at: '',
+          }),
+        })
+      );
       expect(subscriptionManager.update).toHaveBeenNthCalledWith(
         2,
         redundantSubscription.id,

libs/payments/cart/src/lib/checkout.service.ts

@@ -812,6 +812,7 @@ export class CheckoutService {
         proration_behavior: 'always_invoice',
         payment_behavior: 'default_incomplete',
         metadata: {
+          [STRIPE_SUBSCRIPTION_METADATA.CancelledForCustomerAt]: '',
           // Note: These fields are due to missing Fivetran support on Stripe multi-currency plans
           [STRIPE_SUBSCRIPTION_METADATA.Amount]: unitAmountForCurrency,
           [STRIPE_SUBSCRIPTION_METADATA.Currency]: cart.currency,