fix(adminui): reject AbLegacy array length >256 at author-time (review I-3)

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

@@ -99,4 +99,34 @@ public sealed class AbLegacyTagConfigModelTests
 
         json.ShouldContain("\"deviceHostAddress\":\"ab://host\"");
     }
+
+    // Array-length cap (review I-3): authored arrayLength > 256 on an AbLegacy array tag must be
+    // rejected at author-time to prevent a Part 6 §5.3.2 ArrayDimensions mismatch at runtime.
+
+    [Fact]
+    public void Validate_returns_error_when_array_length_exceeds_256()
+    {
+        var m = AbLegacyTagConfigModel.FromJson("""{"address":"N7:0","isArray":true,"arrayLength":300}""");
+
+        var error = m.Validate();
+
+        error.ShouldNotBeNull();
+        error.ShouldContain("256");
+    }
+
+    [Fact]
+    public void Validate_returns_null_when_array_length_exactly_256()
+    {
+        var m = AbLegacyTagConfigModel.FromJson("""{"address":"N7:0","isArray":true,"arrayLength":256}""");
+
+        m.Validate().ShouldBeNull();
+    }
+
+    [Fact]
+    public void Validate_returns_null_for_scalar_tag_regardless_of_no_array_length()
+    {
+        var m = AbLegacyTagConfigModel.FromJson("""{"address":"N7:0"}""");
+
+        m.Validate().ShouldBeNull();
+    }
 }