feat(cloud-billing): cancel subscriptions at period end on deletion mark (#13580)

## How to reproduce
In Chatwoot Cloud, mark an account for deletion from account settings
while the account has an active Stripe subscription. Before this change,
deletion marking did not explicitly mark subscriptions to stop renewing
at period end.

## What changed
This PR adds `Enterprise::Billing::CancelCloudSubscriptionsService` and
calls it from the delete action path in
`Enterprise::Api::V1::AccountsController`. The service lists only active
Stripe subscriptions for the customer and sets `cancel_at_period_end:
true` when needed. The account deletion schedule remains unchanged
(existing static 7-day behavior), and Stripe deleted-event fallback
behavior remains unchanged.

## How this was tested
Added and updated specs:
-
`spec/enterprise/services/enterprise/billing/cancel_cloud_subscriptions_service_spec.rb`
-
`spec/enterprise/controllers/enterprise/api/v1/accounts_controller_spec.rb`

Executed:
- `bundle exec rspec
spec/enterprise/services/enterprise/billing/cancel_cloud_subscriptions_service_spec.rb`
- `bundle exec rspec
spec/enterprise/controllers/enterprise/api/v1/accounts_controller_spec.rb:363`

spec/enterprise/services/enterprise/billing/cancel_cloud_subscriptions_service_spec.rb

@@ -0,0 +1,51 @@
+require 'rails_helper'
+
+RSpec.describe Enterprise::Billing::CancelCloudSubscriptionsService do
+  subject(:service) { described_class.new(account: account) }
+
+  let(:account) { create(:account, custom_attributes: custom_attributes) }
+  let(:custom_attributes) { { 'stripe_customer_id' => 'cus_123' } }
+
+  describe '#perform' do
+    context 'when deployment is not cloud' do
+      it 'does not call stripe subscriptions api' do
+        allow(ChatwootApp).to receive(:chatwoot_cloud?).and_return(false)
+        allow(Stripe::Subscription).to receive(:list)
+
+        service.perform
+
+        expect(Stripe::Subscription).not_to have_received(:list)
+      end
+    end
+
+    context 'when stripe customer id is missing' do
+      let(:custom_attributes) { {} }
+
+      it 'does not call stripe subscriptions api' do
+        allow(ChatwootApp).to receive(:chatwoot_cloud?).and_return(true)
+        allow(Stripe::Subscription).to receive(:list)
+
+        service.perform
+
+        expect(Stripe::Subscription).not_to have_received(:list)
+      end
+    end
+
+    context 'when account is cloud with active subscriptions' do
+      let(:subscription_response) { Struct.new(:data).new([sub_1, sub_2]) }
+      let(:sub_1) { instance_double(Stripe::Subscription, id: 'sub_1', cancel_at_period_end: false) }
+      let(:sub_2) { instance_double(Stripe::Subscription, id: 'sub_2', cancel_at_period_end: true) }
+
+      it 'marks only active subscriptions that are not yet set to cancel at period end' do
+        allow(ChatwootApp).to receive(:chatwoot_cloud?).and_return(true)
+        allow(Stripe::Subscription).to receive(:list).and_return(subscription_response)
+        allow(Stripe::Subscription).to receive(:update)
+
+        service.perform
+
+        expect(Stripe::Subscription).to have_received(:list).with(customer: 'cus_123', status: 'active', limit: 100)
+        expect(Stripe::Subscription).to have_received(:update).with('sub_1', cancel_at_period_end: true).once
+      end
+    end
+  end
+end