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

src/ScadaLink.NotificationService/NotificationDeliveryService.cs

@@ -315,8 +315,6 @@ public class NotificationDeliveryService : INotificationDeliveryService
 
             var bccAddresses = recipients.Select(r => r.EmailAddress).ToList();
             await smtp.SendAsync(config.FromAddress, bccAddresses, subject, body, cancellationToken);
-
-            await smtp.DisconnectAsync(cancellationToken);
         }
         catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
         {
@@ -334,6 +332,23 @@ public class NotificationDeliveryService : INotificationDeliveryService
         // catch filters (SmtpPermanentException / IsTransientSmtpError) handle them.
         finally
         {
+            // NS-010: always tear the connection down, regardless of outcome. The
+            // SMTP QUIT used to run only on the success path inside the try block,
+            // so a failed Connect/Authenticate/Send left an open, authenticated
+            // connection until finalization reclaimed the socket — exhausting the
+            // server's connection slots under sustained transient failures.
+            // Disconnect is best-effort: a disconnect failure (e.g. the connection
+            // is already dead) must not mask the original delivery exception.
+            try
+            {
+                await smtp.DisconnectAsync(cancellationToken);
+            }
+            catch (Exception disconnectEx)
+            {
+                _logger.LogDebug(
+                    "Ignoring SMTP disconnect failure during cleanup: {Reason}", disconnectEx.Message);
+            }
+
             // NS-007: always release the concurrency slot, even on failure.
             limiter.Release();
         }
@@ -399,12 +414,3 @@ public class NotificationDeliveryService : INotificationDeliveryService
         return ClassifySmtpError(ex, cancellationToken) == SmtpErrorClass.Transient;
     }
 }
-
-/// <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) { }
-}