test(billing): add unit tests for getOrSetDefaultPaymentMethod function

- 10 new tests with Stripe API mocking (customers.retrieve/update)
- Tests existing default reuse, setting new default, invalid default replacement
- Tests expanded payment method object handling
- Tests deleted customer handling
- Tests logging behavior (debug, info, warn)
- Tests graceful error handling when update fails
- Tests log context propagation
- Total: 61 tests for auto-topup-helpers (was 51, now 61)

Author: brandonkachen