fix: skip pay call if invoice already paid after finalize (#13924)

## Description

When a customer downgrades from Enterprise to Business, they may retain
unused Stripe credit balance. During an AI credits topup,
Stripe::Invoice.finalize_invoice auto-applies that credit balance to the
invoice. If the credit balance fully covers the invoice amount, Stripe
marks it as paid immediately upon finalization. Calling
Stripe::Invoice.pay on an already-paid invoice throws an error, breaking
the topup flow.
This fix retrieves the invoice status after finalization and skips the
pay call if Stripe has already settled it via credits.

## Type of change

Please delete options that are not relevant.

- [ ] Bug fix (non-breaking change which fixes an issue)

## How Has This Been Tested?

Tested against Stripe test mode with the following scenarios:

- Full credit balance payment: Customer has enough Stripe credit balance
to cover the entire invoice. Invoice is marked paid after
finalize_invoice — pay is correctly skipped. Credits are fulfilled
successfully.
- Partial credit balance payment: Customer has some Stripe credit
balance but not enough to cover the full amount. Invoice remains open
after finalization — pay is called and charges the remaining amount to
the default payment method. Credits are fulfilled successfully.
- Zero credit balance (normal payment): Customer has no Stripe credit
balance. Invoice remains open after finalization — pay charges the full
amount. Credits are fulfilled successfully.


## Checklist:

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

spec/enterprise/controllers/enterprise/api/v1/accounts_controller_spec.rb

@@ -281,6 +281,7 @@ RSpec.describe 'Enterprise Billing APIs', type: :request do
         allow(Stripe::Invoice).to receive(:create).and_return(stripe_invoice)
         allow(Stripe::InvoiceItem).to receive(:create)
         allow(Stripe::Invoice).to receive(:finalize_invoice)
+        allow(Stripe::Invoice).to receive(:retrieve).and_return(Struct.new(:status).new('open'))
         allow(Stripe::Invoice).to receive(:pay)
         allow(Stripe::Billing::CreditGrant).to receive(:create)
       end

spec/enterprise/services/enterprise/billing/topup_checkout_service_spec.rb

@@ -24,6 +24,7 @@ describe Enterprise::Billing::TopupCheckoutService do
     allow(Stripe::Invoice).to receive(:create).and_return(stripe_invoice)
     allow(Stripe::InvoiceItem).to receive(:create)
     allow(Stripe::Invoice).to receive(:finalize_invoice)
+    allow(Stripe::Invoice).to receive(:retrieve).and_return(Struct.new(:status).new('open'))
     allow(Stripe::Invoice).to receive(:pay)
     allow(Stripe::Billing::CreditGrant).to receive(:create)
   end
@@ -58,5 +59,19 @@ describe Enterprise::Billing::TopupCheckoutService do
         expect(error.message).to eq(I18n.t('errors.topup.plan_not_eligible'))
       end
     end
+
+    it 'calls pay when invoice is open after finalization' do
+      service.create_checkout_session(credits: 1000)
+
+      expect(Stripe::Invoice).to have_received(:pay).with('inv_test123')
+    end
+
+    it 'skips pay when invoice is already paid via Stripe credits' do
+      allow(Stripe::Invoice).to receive(:retrieve).and_return(Struct.new(:status).new('paid'))
+
+      service.create_checkout_session(credits: 1000)
+
+      expect(Stripe::Invoice).not_to have_received(:pay)
+    end
   end
 end