fix(sms): repair S1 build breaks — null-filter EmailAddress projections + SiteNotificationRepository SMS stubs

S1 made NotificationRecipient.EmailAddress nullable + added SmsConfiguration
and four INotificationRepository SMS methods, breaking compilation beyond the
intentionally-deferred central NotificationRepository.

Fix 1 (CS8620/CS8604 nullable EmailAddress projections, email-only paths):
- NotificationOutbox EmailNotificationDeliveryAdapter: filter non-null emails
- DeploymentManager ArtifactDeploymentService: filter non-null emails
- Transport EntitySerializer: filter non-null emails into NotificationRecipientDto

Fix 2 (CS0535): stub the four SMS-config methods on SiteRuntime
SiteNotificationRepository (central-only — NotSupportedException, matching the
existing 'Managed via artifact deployment from Central' write-path pattern).

Doc nits: reword NotificationRecipient private ctor and SmsConfiguration.AuthToken
comments.

The central ConfigurationDatabase.NotificationRepository compile break is left
as-is (S2 implements those four methods).

src/ZB.MOM.WW.ScadaBridge.NotificationOutbox/Delivery/EmailNotificationDeliveryAdapter.cs

@@ -106,7 +106,10 @@ public sealed class EmailNotificationDeliveryAdapter : INotificationDeliveryAdap
             return DeliveryOutcome.Permanent(addressError);
         }
 
-        var recipientAddresses = recipients.Select(r => r.EmailAddress).ToList();
+        var recipientAddresses = recipients
+            .Where(r => r.EmailAddress is not null)
+            .Select(r => r.EmailAddress!)
+            .ToList();
 
         try
         {