refactor(adminui): harden SetFullName (reject blank ref) + cover alarm-flag re-pick

Code-review nits: SetFullName now throws on a blank reference (was silently
persisting FullName:null → silent deploy-time bind failure), and a new test
covers the alarm-typed re-pick combo (SeedDefaultAlarm over an already-edited
alarm leaves it intact).

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Uns/TagEditors/TagConfigJson.cs

@@ -54,6 +54,9 @@ public static class TagConfigJson
     /// </summary>
     public static string SetFullName(string? json, string fullName)
     {
+        // A blank reference is never a valid Galaxy bind — surface it loudly rather than persisting
+        // a poisoned "FullName": null/"" that would silently fail to bind at deploy time.
+        ArgumentException.ThrowIfNullOrWhiteSpace(fullName);
         var o = ParseOrNew(json);
         o["FullName"] = JsonValue.Create(fullName);
         return Serialize(o);