[TEST] Add integration tests for payment webhook endpoints

[aniebietafia]

Problem

Stripe, PayPal, and Paystack webhooks are responsible for updating user credits and subscription statuses. These endpoints process raw JSON payloads, compute signatures, and trigger database modifications. They currently lack integration test coverage.

Proposed Solution

Create integration tests that mock external payment gateway requests. Send raw payload signatures to webhook endpoints using Spring's MockMvc and verify that the database correctly updates subscription and transactional records.

User Stories

• As an operator, I want webhook endpoints verified automatically to guarantee that subscriptions update correctly upon receipt of payment events.

Acceptance Criteria

• Webhook integration tests mock signature validation.
• Mock transactions execute successfully and modify test database records.

Tasks

• Create StripeWebhookIntegrationTest.java, PaystackWebhookIntegrationTest.java, and PayPalWebhookIntegrationTest.java.
• Draft sample event payloads.
• Assert status modifications in test database fixtures.