fix(notification-service): resolve NotificationService-010,011,012 — disconnect SMTP on failure, relocate exception type, OAuth2/token-cache test coverage

src/ScadaLink.NotificationService/SmtpPermanentException.cs

@@ -0,0 +1,10 @@
+namespace ScadaLink.NotificationService;
+
+/// <summary>
+/// Signals a permanent SMTP failure (5xx) that should not be retried.
+/// </summary>
+public class SmtpPermanentException : Exception
+{
+    public SmtpPermanentException(string message, Exception? innerException = null)
+        : base(message, innerException) { }
+}