perf: update the logic to purchase credits (#12998)

## Description

- Replaces Stripe Checkout session flow with direct card charging for AI
credit top-ups
- Adds a two-step confirmation modal (select package → confirm purchase)
for better UX
- Creates Stripe invoice directly and charges the customer's default
payment method immediately

## Type of change

- [ ] New feature (non-breaking change which adds functionality)

## How Has This Been Tested?

- Using the specs
- UI manual test cases

<img width="945" height="580" alt="image"
src="https://github.com/user-attachments/assets/52bdad46-cd0e-4927-b13f-54c6b6353bcc"
/>

<img width="945" height="580" alt="image"
src="https://github.com/user-attachments/assets/231bc7e9-41ac-440d-a93d-cba45a4d3e3e"
/>


## 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

---------

Co-authored-by: Shivam Mishra <scm.mymail@gmail.com>

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

@@ -0,0 +1,60 @@
+require 'rails_helper'
+
+describe Enterprise::Billing::TopupCheckoutService do
+  subject(:service) { described_class.new(account: account) }
+
+  let(:account) { create(:account) }
+  let(:stripe_customer_id) { 'cus_test123' }
+  let(:invoice_settings) { Struct.new(:default_payment_method).new('pm_test') }
+  let(:stripe_customer) { Struct.new(:invoice_settings, :default_source).new(invoice_settings, nil) }
+  let(:stripe_invoice) { Struct.new(:id).new('inv_test123') }
+
+  before do
+    create(:installation_config, name: 'CHATWOOT_CLOUD_PLANS', value: [
+             { 'name' => 'Hacker', 'product_id' => ['prod_hacker'], 'price_ids' => ['price_hacker'] },
+             { 'name' => 'Business', 'product_id' => ['prod_business'], 'price_ids' => ['price_business'] }
+           ])
+
+    account.update!(
+      custom_attributes: { plan_name: 'Business', stripe_customer_id: stripe_customer_id },
+      limits: { 'captain_responses' => 500 }
+    )
+
+    allow(Stripe::Customer).to receive(:retrieve).and_return(stripe_customer)
+    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(:pay)
+    allow(Stripe::Billing::CreditGrant).to receive(:create)
+  end
+
+  describe '#create_checkout_session' do
+    it 'successfully processes topup and returns correct response' do
+      result = service.create_checkout_session(credits: 1000)
+
+      expect(result[:credits]).to eq(1000)
+      expect(result[:amount]).to eq(20.0)
+      expect(result[:currency]).to eq('usd')
+    end
+
+    it 'updates account limits after successful topup' do
+      service.create_checkout_session(credits: 1000)
+
+      expect(account.reload.limits['captain_responses']).to eq(1500)
+    end
+
+    it 'raises error for invalid credits' do
+      expect do
+        service.create_checkout_session(credits: 500)
+      end.to raise_error(Enterprise::Billing::TopupCheckoutService::Error)
+    end
+
+    it 'raises error when account is on free plan' do
+      account.update!(custom_attributes: { plan_name: 'Hacker', stripe_customer_id: stripe_customer_id })
+
+      expect do
+        service.create_checkout_session(credits: 1000)
+      end.to raise_error(Enterprise::Billing::TopupCheckoutService::Error)
+    end
+  end
+end

spec/enterprise/services/enterprise/billing/topup_fulfillment_service_spec.rb

@@ -0,0 +1,36 @@
+require 'rails_helper'
+
+describe Enterprise::Billing::TopupFulfillmentService do
+  subject(:service) { described_class.new(account: account) }
+
+  let(:account) { create(:account) }
+  let(:stripe_customer_id) { 'cus_test123' }
+
+  before do
+    account.update!(
+      custom_attributes: { stripe_customer_id: stripe_customer_id },
+      limits: { 'captain_responses' => 1000 }
+    )
+    allow(Stripe::Billing::CreditGrant).to receive(:create)
+  end
+
+  describe '#fulfill' do
+    it 'adds credits to account limits' do
+      service.fulfill(credits: 1000, amount_cents: 2000, currency: 'usd')
+
+      expect(account.reload.limits['captain_responses']).to eq(2000)
+    end
+
+    it 'creates a Stripe credit grant' do
+      service.fulfill(credits: 1000, amount_cents: 2000, currency: 'usd')
+
+      expect(Stripe::Billing::CreditGrant).to have_received(:create).with(
+        hash_including(
+          customer: stripe_customer_id,
+          name: 'Topup: 1000 credits',
+          category: 'paid'
+        )
+      )
+    end
+  end
+end