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>

enterprise/app/controllers/enterprise/api/v1/accounts_controller.rb

@@ -59,10 +59,15 @@ class Enterprise::Api::V1::AccountsController < Api::BaseController
     return render json: { error: I18n.t('errors.topup.credits_required') }, status: :unprocessable_entity if params[:credits].blank?
 
     service = Enterprise::Billing::TopupCheckoutService.new(account: @account)
-    redirect_url = service.create_checkout_session(credits: params[:credits].to_i)
-    render json: { redirect_url: redirect_url }
-  rescue Enterprise::Billing::TopupCheckoutService::Error => e
-    Rails.logger.error("Topup checkout failed for account #{@account.id}: #{e.message}")
+    result = service.create_checkout_session(credits: params[:credits].to_i)
+
+    @account.reload
+    render json: result.merge(
+      id: @account.id,
+      limits: @account.limits,
+      custom_attributes: @account.custom_attributes
+    )
+  rescue Enterprise::Billing::TopupCheckoutService::Error, Stripe::StripeError => e
     render_could_not_create_error(e.message)
   end