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.DeploymentManager/ArtifactDeploymentService.cs

@@ -179,7 +179,7 @@ public class ArtifactDeploymentService
 
         // Map notification lists
         var notificationListArtifacts = notificationLists.Select(nl =>
-            new NotificationListArtifact(nl.Name, nl.Recipients.Select(r => r.EmailAddress).ToList())).ToList();
+            new NotificationListArtifact(nl.Name, nl.Recipients.Where(r => r.EmailAddress is not null).Select(r => r.EmailAddress!).ToList())).ToList();
 
         // Map SMTP configurations — use Host as the artifact name (matches SQLite PK on site)
         var smtpArtifacts = smtpConfigurations.Select(smtp =>