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.ConfigurationDatabase/Configurations/NotificationConfiguration.cs

@@ -73,8 +73,10 @@ public class SmsConfigurationConfiguration : IEntityTypeConfiguration<SmsConfigu
         builder.Property(s => s.AuthToken)
             .HasMaxLength(8000);
 
+        // Optional: a Twilio Messaging-Service-only config has no From number (a valid
+        // config carries a FromNumber and/or a MessagingServiceSid).
         builder.Property(s => s.FromNumber)
-            .IsRequired()
+            .IsRequired(false)
             .HasMaxLength(32);
 
         builder.Property(s => s.MessagingServiceSid)

src/ZB.MOM.WW.ScadaBridge.ConfigurationDatabase/Migrations/20260619191520_SmsFromNumberOptional.cs

@@ -0,0 +1,33 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Migrations
+{
+    /// <summary>
+    /// SMS code-review (CentralUI-NNN / UI-Med-2): makes <c>SmsConfigurations.FromNumber</c>
+    /// optional. A Twilio Messaging-Service-only config supplies a MessagingServiceSid instead
+    /// of a From number; the design doc, the delivery adapter, and now the UI/CLI treat the two
+    /// as either-or, so the column must allow NULL.
+    ///
+    /// ALTER COLUMN to NULL is naturally idempotent (re-running re-applies the same shape).
+    /// </summary>
+    public partial class SmsFromNumberOptional : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.Sql("ALTER TABLE [SmsConfigurations] ALTER COLUMN [FromNumber] nvarchar(32) NULL;");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            // Reversing to NOT NULL requires backfilling NULLs first. Unlike the recipient
+            // EmailAddress backfill, '' is a harmless reversal here: a Messaging-Service-only
+            // config keeps its MessagingServiceSid and stays deliverable, so no data is lost.
+            migrationBuilder.Sql("UPDATE [SmsConfigurations] SET [FromNumber] = '' WHERE [FromNumber] IS NULL;");
+            migrationBuilder.Sql("ALTER TABLE [SmsConfigurations] ALTER COLUMN [FromNumber] nvarchar(32) NOT NULL;");
+        }
+    }
+}

src/ZB.MOM.WW.ScadaBridge.ConfigurationDatabase/Migrations/ScadaBridgeDbContextModelSnapshot.cs

@@ -871,7 +871,6 @@ namespace ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Migrations
                         .HasColumnType("int");
 
                     b.Property<string>("FromNumber")
-                        .IsRequired()
                         .HasMaxLength(32)
                         .HasColumnType("nvarchar(32)");