refactor: Abstract stripe API calls into their own modules

[sashko9807]

This pull request introduces a new StripeApiClient gateway class to centralize and standardize all Stripe API interactions in the codebase. It updates the Stripe integration to ensure every Stripe SDK call goes through this gateway, improving maintainability, testability, and permission management. The PR also refactors tests and documentation to reflect this new structure.

Stripe integration improvements:

• Added the StripeApiClient class (apps/api/src/stripe/stripe-api-client.ts) as a single gateway for all Stripe SDK calls, enabling centralized permission probing, logging, and error handling. All direct Stripe SDK calls should now go through this class.
• Updated the Stripe module (apps/api/src/stripe/stripe.module.ts) to provide StripeApiClient as a dependency, ensuring it is available for injection throughout the application. [1] [2]

Test refactoring:

• Refactored tests in stripe.controller.spec.ts, recurring-donation.controller.spec.ts, and stripe-payment.service.spec.ts to use a deep mock of StripeApiClient instead of mocking the Stripe SDK directly. This ensures tests are aligned with the new gateway and simplifies mocking Stripe calls. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

Developer documentation:

• Greatly expanded and clarified the Stripe testing instructions in TESTING.md, including first-time setup, recommended use of restricted keys, and a new workflow for verifying required permissions using the gateway. [1] [2]

These changes make Stripe integration more robust, easier to audit, and safer to extend.

[github-actions]

✅ Tests will run for this PR. Once they succeed it can be merged.