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.

tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/Pages/SmsConfigurationPageTests.cs

@@ -126,6 +126,39 @@ public class SmsConfigurationPageTests : BunitContext
         });
     }
 
+    [Fact]
+    public void SavingNewConfig_MessagingServiceSidOnly_NoFromNumber_Saves()
+    {
+        // UI-Med-2: a Twilio Messaging-Service-only config is valid with no From number.
+        // The either-or validation must accept it and persist a null FromNumber.
+        var repo = RepoWith();
+        Services.AddSingleton(repo);
+        WireAuth();
+
+        var cut = Render<SmsConfigurationPage>();
+        cut.WaitForState(() => cut.Markup.Contains("No SMS configuration set."));
+
+        cut.FindAll("button").First(b => b.TextContent.Contains("Add SMS configuration")).Click();
+
+        // Account SID + Messaging Service SID, leaving From Number (index 1) blank.
+        cut.FindAll("input[type=text]")[0].Change("ACmsg_account");        // Account SID
+        cut.FindAll("input[type=text]")[2].Change("MGmessaging_service");  // Messaging Service SID
+        cut.FindAll("input[type=password]").First().Change("new-token");
+
+        cut.FindAll("button").First(b => b.TextContent.Contains("Save")).Click();
+
+        cut.WaitForAssertion(() =>
+        {
+            repo.Received().AddSmsConfigurationAsync(
+                Arg.Is<SmsConfiguration>(c =>
+                    c.AccountSid == "ACmsg_account" &&
+                    c.FromNumber == null &&
+                    c.MessagingServiceSid == "MGmessaging_service" &&
+                    c.AuthToken == "new-token"));
+            repo.Received().SaveChangesAsync();
+        });
+    }
+
     [Fact]
     public void SavingEdit_WithBlankAuthToken_PreservesExistingToken()
     {