test(billing): use fake timers in checkout webhook unit test (slow regression)

[PierreBrisorgueil]

Context

PR for Phase 2 audit fix (refund correlation retry — branch fix/billing-refund-correlation-retry) introduces retryWithBackoff around the PaymentIntent metadata backfill call. The existing test should not throw when paymentIntents.update fails (non-fatal fallback) at modules/billing/tests/billing.webhook.checkout.unit.tests.js:286 now incurs ~600ms of real wall-clock delays per run because :

1. Retry wraps stripe.paymentIntents.update with backoff 200ms → 400ms → final
2. The existing test mocks Stripe to throw, so all 3 retries fire with real setTimeout
3. Test still passes (the assertion is .resolves.toBeUndefined()) but is now slow

Acceptance

• Switch the test to jest.useFakeTimers() (or equivalent) + jest.runAllTimersAsync()
• Test still passes
• No real-timer delay incurred

Source

Discovered by spec reviewer on PR for Phase 2 audit fix (audit P1 — 2026-05-21).