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);

tests/Server/ZB.MOM.WW.OtOpcUa.AdminUI.Tests/Uns/GalaxyAddressRepickMergeTests.cs

@@ -75,6 +75,31 @@ public sealed class GalaxyAddressRepickMergeTests
         merged.ShouldEndWith("}");
     }
 
+    [Fact]
+    public void Repick_with_alarm_flag_does_not_overwrite_existing_alarm_object()
+    {
+        // Mirrors the handler's alarm-typed re-pick: SeedDefaultAlarm(SetFullName(existing, addr)).
+        // The operator authored an alarm, then re-picks an alarm-typed Galaxy attribute — the
+        // SeedDefaultAlarm no-op guard must leave the edited alarm intact, not re-seed a default.
+        const string current =
+            """{"FullName":"Pump_001.OldAttr","alarm":{"alarmType":"LimitAlarm","severity":250,"historizeToAveva":false}}""";
+
+        var merged = NativeAlarmModel.SeedDefaultAlarm(TagConfigJson.SetFullName(current, "Pump_001.NewAttr"));
+
+        ReadFullName(merged).ShouldBe("Pump_001.NewAttr");
+        var alarm = NativeAlarmModel.FromJson(merged);
+        alarm.IsAlarm.ShouldBeTrue();
+        alarm.AlarmType.ShouldBe("LimitAlarm");
+        alarm.Severity.ShouldBe(250);
+        alarm.HistorizeToAveva.ShouldBe(false);
+    }
+
+    [Fact]
+    public void SetFullName_rejects_a_blank_reference()
+    {
+        Should.Throw<ArgumentException>(() => TagConfigJson.SetFullName("""{"FullName":"x"}""", "  "));
+    }
+
     // Mirror of TagModal.ReadFullName — extracts FullName from a Galaxy TagConfig blob.
     private static string ReadFullName(string json)
     {