Add unit test for HtmlElementErrorEventArgs.cs file (#13512)

Add unit test for HtmlElementErrorEventArgs file

Author: Philip-Wang01

src/test/unit/System.Windows.Forms/System/Windows/Forms/HtmlElementErrorEventArgsTests.cs

@@ -0,0 +1,45 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Windows.Forms.Tests;
+
+public class HtmlElementErrorEventArgsTests
+{
+    [Fact]
+    public void HtmlElementErrorEventArgs_InitializesProperties()
+    {
+        string description = "Script error";
+        string urlString = "https://example.com/page";
+        int lineNumber = 42;
+
+        HtmlElementErrorEventArgs args = new(description, urlString, lineNumber);
+
+        args.Description.Should().Be(description);
+        args.LineNumber.Should().Be(lineNumber);
+        args.Url.Should().Be(new Uri(urlString));
+        args.Handled.Should().BeFalse();
+        args.Handled = true;
+        args.Handled.Should().BeTrue();
+    }
+
+    [Fact]
+    public void HtmlElementErrorEventArgs_Url_CachesUriInstance()
+    {
+        HtmlElementErrorEventArgs args = new("desc", "https://test", 1);
+
+        Uri url1 = args.Url;
+        Uri url2 = args.Url;
+
+        url1.Should().BeSameAs(url2);
+    }
+
+    [Fact]
+    public void HtmlElementErrorEventArgs_Url_ThrowsOnInvalidUri()
+    {
+        HtmlElementErrorEventArgs args = new("desc", "not a uri", 1);
+
+        Action action = () => _ = args.Url;
+
+        action.Should().Throw<UriFormatException>();
+    }
+}