FU1 of the Notification Outbox follow-ups. EmailNotificationDeliveryAdapter
carried verbatim private copies of credential redaction, SMTP error
classification, and address validation because the NotificationService
helpers were internal. This eliminates the divergence risk by promoting the
helpers to public and deleting the adapter's copies.
- CredentialRedactor: internal -> public.
- Extract SmtpErrorClassifier + SmtpErrorClass enum into a new public static
class; NotificationDeliveryService now routes classification through it
(behavior unchanged). Adds focused SmtpErrorClassifierTests.
- NotificationDeliveryService.ValidateAddresses: internal -> public; the
adapter calls it directly.
- EmailNotificationDeliveryAdapter: deleted ScrubCredentials, ClassifySmtpError,
SmtpErrorClass, IsTransientSmtpError and ValidateAddresses copies.
No InternalsVisibleTo hack — specific helpers promoted to public. Both test
suites green; full solution builds clean.
