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.Commons/Entities/Notifications/NotificationRecipient.cs

@@ -71,8 +71,9 @@ public class NotificationRecipient
     }
 
     /// <summary>
-    /// Parameterless constructor used by EF Core materialization and the SMS factory, where the
-    /// contact field is assigned via property setters rather than constructor parameters.
+    /// Private parameterless constructor that backs the <see cref="ForSms"/> factory path,
+    /// where the contact field is assigned via property setters rather than constructor
+    /// parameters — without exposing a half-initialized public constructor to callers.
     /// </summary>
     private NotificationRecipient()
     {

src/ZB.MOM.WW.ScadaBridge.Commons/Entities/Notifications/SmsConfiguration.cs

@@ -6,7 +6,10 @@ public class SmsConfiguration
     public int Id { get; set; }
     /// <summary>Gets or sets the Twilio Account SID.</summary>
     public string AccountSid { get; set; }
-    /// <summary>Gets or sets the Twilio Auth Token (secret), or null when not applicable.</summary>
+    /// <summary>
+    /// Gets or sets the Twilio Auth Token (secret). Stored encrypted; null only transiently
+    /// during configuration, never a valid production value.
+    /// </summary>
     public string? AuthToken { get; set; }
     /// <summary>Gets or sets the sender phone number (E.164) placed in the From field.</summary>
     public string FromNumber { get; set; }