Adds some specific serialization test cases for the ToolStripMenuItemCodeDomSerializer class

Related #10773
Related #12883

- After the merging of #12883, [there was a request](#12883 (comment)) for some specific serialization test cases of the `ToolStripMenuItemCodeDomSerializer` class.

- Adds some specific serialization test cases for the `ToolStripMenuItemCodeDomSerializer` class.

- None

- Minimal

- Unit tests

- 10.0.100-preview.3.25167.3

Author: Ricardo Bossan (BEYONDSOFT CONSULTING INC) (from Dev Box)

src/System.Windows.Forms.Design/tests/UnitTests/System/Windows/Forms/Design/ToolStripMenuItemCodeDomSerializerTests.cs

@@ -1,9 +1,10 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using Moq;
 using System.ComponentModel;
 using System.ComponentModel.Design.Serialization;
+using System.Drawing;
 
 namespace System.Windows.Forms.Design.Tests;
 
@@ -73,4 +74,80 @@ public void Serialize_CallsBaseSerializerForNonDummyItem()
         result.Should().NotBeNull();
         _mockBaseSerializer.Verify(s => s.Serialize(_mockManager.Object, nonDummyItem), Times.Once);
     }
+
+    [Fact]
+    public void Serialize_SerializesDropDownItemsCorrectly()
+    {
+        _mockManager.Setup(m => m.GetSerializer(typeof(ToolStripMenuItem), typeof(CodeDomSerializer)))
+                    .Returns(_mockBaseSerializer.Object);
+
+        ToolStripMenuItem parentItem = new() { Text = "Edit" };
+        ToolStripMenuItem childItem = new() { Text = "Undo" };
+        parentItem.DropDownItems.Add(childItem);
+
+        _mockBaseSerializer.Setup(s => s.Serialize(_mockManager.Object, parentItem))
+                           .Returns(new { parentItem.Text, DropDownItems = new[] { new { childItem.Text } } });
+
+        object? result = _serializer.Serialize(_mockManager.Object, parentItem);
+
+        result.Should().NotBeNull();
+        result.Should().BeEquivalentTo(new { Text = "Edit", DropDownItems = new[] { new { Text = "Undo" } } });
+        _mockBaseSerializer.Verify(s => s.Serialize(_mockManager.Object, parentItem), Times.Once);
+    }
+
+    [Fact]
+    public void Serialize_SerializesEnabledPropertyCorrectly()
+    {
+        _mockManager.Setup(m => m.GetSerializer(typeof(ToolStripMenuItem), typeof(CodeDomSerializer)))
+                    .Returns(_mockBaseSerializer.Object);
+
+        ToolStripMenuItem item = new() { Enabled = false };
+        _mockBaseSerializer.Setup(s => s.Serialize(_mockManager.Object, item))
+                           .Returns(new { item.Enabled });
+
+        object? result = _serializer.Serialize(_mockManager.Object, item);
+
+        result.Should().NotBeNull();
+        result.Should().BeEquivalentTo(new { Enabled = false });
+        _mockBaseSerializer.Verify(s => s.Serialize(_mockManager.Object, item), Times.Once);
+    }
+
+    [Fact]
+    public void Serialize_SerializesCheckedAndTextCorrectly()
+    {
+        _mockManager.Setup(m => m.GetSerializer(typeof(ToolStripMenuItem), typeof(CodeDomSerializer)))
+                    .Returns(_mockBaseSerializer.Object);
+
+        ToolStripMenuItem parentItem = new() { Text = "File", Checked = true };
+        ToolStripMenuItem childItem = new() { Text = "New", Checked = false };
+        parentItem.DropDownItems.Add(childItem);
+
+        _mockBaseSerializer.Setup(s => s.Serialize(_mockManager.Object, parentItem))
+                           .Returns(new { parentItem.Text, parentItem.Checked, DropDownItems = new[] { new { childItem.Text, childItem.Checked } } });
+
+        object? result = _serializer.Serialize(_mockManager.Object, parentItem);
+
+        result.Should().NotBeNull();
+        result.Should().BeEquivalentTo(new { Text = "File", Checked = true, DropDownItems = new[] { new { Text = "New", Checked = false } } });
+        _mockBaseSerializer.Verify(s => s.Serialize(_mockManager.Object, parentItem), Times.Once);
+    }
+
+    [Fact]
+    public void Serialize_SerializesImagePropertyCorrectly()
+    {
+        _mockManager.Setup(m => m.GetSerializer(typeof(ToolStripMenuItem), typeof(CodeDomSerializer)))
+                    .Returns(_mockBaseSerializer.Object);
+
+        Image testImage = new Bitmap(1, 1);
+        ToolStripMenuItem item = new() { Image = testImage };
+
+        _mockBaseSerializer.Setup(s => s.Serialize(_mockManager.Object, item))
+                           .Returns(new { item.Image });
+
+        object? result = _serializer.Serialize(_mockManager.Object, item);
+
+        result.Should().NotBeNull();
+        result.Should().BeEquivalentTo(new { Image = testImage });
+        _mockBaseSerializer.Verify(s => s.Serialize(_mockManager.Object, item), Times.Once);
+    }
 }