feat(sms): make FromNumber optional — support Twilio Messaging-Service-only configs (UI-Med-2)

Code-review finding UI-Med-2: the design doc + delivery adapter treat FromNumber and
MessagingServiceSid as either-or, but the entity ctor, EF schema, UI and CLI all hard-
required FromNumber — so a Messaging-Service-only Twilio config (a normal production
setup) could not be created. Bring the implementation into line with the spec:

- Commons: SmsConfiguration.FromNumber -> string? (ctor fromNumber optional);
  UpdateSmsConfigCommand.FromNumber -> string?.
- ConfigurationDatabase: FromNumber.IsRequired(false) + migration SmsFromNumberOptional
  (ALTER COLUMN nullable, idempotent; Down backfills '' — harmless, MsgSid keeps it
  deliverable) + regenerated model snapshot.
- Transport: SmsConfigDto.FromNumber -> string? (round-trips a Messaging-Service-only config).
- CentralUI: form validation requires AccountSid + at-least-one-of(FromNumber, MsgSid);
  nullable create/edit paths; From-number help text.
- CLI: --from-number no longer Required; BuildUpdateSmsConfigCommand validates the either-or.
- Adapter: From branch null-forgiving (guarded by the existing incomplete-config check).

Tests: ManagementActor MsgSid-only persists null FromNumber; CLI MsgSid-only builds +
neither-throws + contract (--from-number not Required); CentralUI MsgSid-only save.

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

@@ -11,8 +11,14 @@ public class SmsConfiguration
     /// 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; }
+    /// <summary>
+    /// Gets or sets the sender phone number (E.164) placed in the From field, or null when
+    /// delivery is via a <see cref="MessagingServiceSid"/> instead. A valid config has a
+    /// FromNumber and/or a MessagingServiceSid; that either-or invariant is enforced at the
+    /// management/UI boundary and again by the delivery adapter (it is not a ctor invariant
+    /// because MessagingServiceSid is a settable property assigned after construction).
+    /// </summary>
+    public string? FromNumber { get; set; }
     /// <summary>Gets or sets the Twilio Messaging Service SID used instead of a From number, or null.</summary>
     public string? MessagingServiceSid { get; set; }
     /// <summary>Gets or sets the Twilio REST API base URL, or null to use the provider default.</summary>
@@ -39,15 +45,17 @@ public class SmsConfiguration
     public TimeSpan RetryDelay { get; set; }
 
     /// <summary>
-    /// Initializes a new <see cref="SmsConfiguration"/> with required fields and sensible defaults
-    /// for the numeric and timeout fields.
+    /// Initializes a new <see cref="SmsConfiguration"/> with the required Account SID and
+    /// sensible defaults for the numeric and timeout fields. <paramref name="fromNumber"/>
+    /// is optional — a Twilio Messaging-Service-only config sets <see cref="MessagingServiceSid"/>
+    /// instead and leaves the From number null.
     /// </summary>
     /// <param name="accountSid">Twilio Account SID.</param>
-    /// <param name="fromNumber">Sender phone number (E.164) for the From field.</param>
-    public SmsConfiguration(string accountSid, string fromNumber)
+    /// <param name="fromNumber">Sender phone number (E.164) for the From field, or null for a Messaging-Service-only config.</param>
+    public SmsConfiguration(string accountSid, string? fromNumber = null)
     {
         AccountSid = accountSid ?? throw new ArgumentNullException(nameof(accountSid));
-        FromNumber = fromNumber ?? throw new ArgumentNullException(nameof(fromNumber));
+        FromNumber = fromNumber;
         ConnectionTimeoutSeconds = 30;
         MaxRetries = 10;
         RetryDelay = TimeSpan.FromMinutes(1);