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.Transport/Serialization/EntityDtos.cs

@@ -220,7 +220,7 @@ public sealed record SmtpConfigDto(
 /// </summary>
 public sealed record SmsConfigDto(
     string AccountSid,
-    string FromNumber,
+    string? FromNumber,
     string? MessagingServiceSid,
     string? ApiBaseUrl,
     int ConnectionTimeoutSeconds,