The test helper's mock email server was missing the getRateLimiter() method
that was added during the rate limiting feature implementation. This caused
all SMTP tests to fail with "getRateLimiter is not a function" error.
Changes:
- Add getRateLimiter() method to mock email server that returns a mock rate limiter
- Update mock rate limiter method signatures to match actual implementation
- Fix TypeScript type issue with auth options by adding explicit casting
