test(emails): add integration smoke test for card expiry warning (NPPD-1524)

Runnable via `wp eval-file tests/integration/card-expiry-warning-smoke.php`.
Covers: cron scheduling, happy-path send, idempotency, clear_sent_flag
handler, new-card re-send, unattached-card filtering, and cleanup.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: kmwilkerson