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>

app/javascript/dashboard/api/enterprise/specs/account.spec.js

@@ -11,6 +11,8 @@ describe('#enterpriseAccountAPI', () => {
     expect(accountAPI).toHaveProperty('delete');
     expect(accountAPI).toHaveProperty('checkout');
     expect(accountAPI).toHaveProperty('toggleDeletion');
+    expect(accountAPI).toHaveProperty('createTopupCheckout');
+    expect(accountAPI).toHaveProperty('getLimits');
   });
 
   describe('API calls', () => {
@@ -59,5 +61,29 @@ describe('#enterpriseAccountAPI', () => {
         { action_type: 'undelete' }
       );
     });
+
+    it('#createTopupCheckout with credits', () => {
+      accountAPI.createTopupCheckout(1000);
+      expect(axiosMock.post).toHaveBeenCalledWith(
+        '/enterprise/api/v1/topup_checkout',
+        { credits: 1000 }
+      );
+    });
+
+    it('#createTopupCheckout with different credit amounts', () => {
+      const creditAmounts = [1000, 2500, 6000, 12000];
+      creditAmounts.forEach(credits => {
+        accountAPI.createTopupCheckout(credits);
+        expect(axiosMock.post).toHaveBeenCalledWith(
+          '/enterprise/api/v1/topup_checkout',
+          { credits }
+        );
+      });
+    });
+
+    it('#getLimits', () => {
+      accountAPI.getLimits();
+      expect(axiosMock.get).toHaveBeenCalledWith('/enterprise/api/v1/limits');
+    });
   });
 });