fix(billing): billing store tests fail — config.api.endPoints.billing is undefined in test env #3738

Problem

Billing store unit tests fail because config.api.endPoints.billing resolves to undefined at test time, producing URLs like http://localhost:3000/api/undefined/subscription instead of http://localhost:3000/api/billing/subscription.

Failing tests (3):

• fetchSubscription > should call correct API endpoint
• createCheckout > should send correct payload with priceId and redirect URLs
• fetchPlans > should call correct API endpoint (similar pattern)

Affected file(s)

• src/modules/billing/tests/billing.store.unit.tests.js
• src/modules/billing/stores/billing.store.js
• Config generation / test config merge

Steps to reproduce

1. Fresh clone of devkit-vue master
2. npm install
3. npm run test:unit
4. Billing store tests fail with URL mismatch (undefined where billing should be)