fix: Uncommented listing and added compilerAssert tests (#543)

## Summary

Had to add an extra class that derived from PdaItem. The code now throws
the error

Fixes #541

src/Chapter12.Tests/Listing12.41.Tests.cs

@@ -0,0 +1,15 @@
+using AddisonWesley.Michaelis.EssentialCSharp.Shared.Tests;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace AddisonWesley.Michaelis.EssentialCSharp.Chapter12.Listing12_41.Tests;
+
+[TestClass]
+public class Listing12
+{
+    [TestMethod]
+    public async Task CannotConvertTypes()
+    {
+        await CompilerAssert.CompileTestTargetFileAsync(
+            new string[] { "CS0030" });
+    }
+}

src/Chapter12/Listing12.41.PreventingCovarianceMaintainsHomogeneity.cs

@@ -1,6 +1,3 @@
-using AddisonWesley.Michaelis.EssentialCSharp.Chapter08.Listing08_05;
-using AddisonWesley.Michaelis.EssentialCSharp.Chapter12.Listing12_11;
-
 namespace AddisonWesley.Michaelis.EssentialCSharp.Chapter12.Listing12_41;
 
 public class Program
@@ -14,12 +11,57 @@ public static void Main()
         Pair<Contact> contacts = new(contact1, contact2);
 
         #region HIGHLIGHT
+        #if COMPILEERROR // EXCLUDE
         // This gives an error: Cannot convert type ...
         // But suppose it did not
-        // IPair<PdaItem> pdaPair = (IPair<PdaItem>) contacts;
+        IPair<PdaItem> pdaPair = (IPair<PdaItem>) contacts;
         // This is perfectly legal, but not type-safe
-        // pair.First = new Address("123 Sesame Street");
+        pdaPair.First = new Address("123 Sesame Street");
+        #endif // COMPILEERROR // EXCLUDE
         #endregion HIGHLIGHT
         #endregion INCLUDE
     }
 }
+
+public class Address : PdaItem
+{
+    public Address(string address) : base(address)
+    {
+    }
+}
+
+public struct Pair<T> : IPair<T>
+{
+    public Pair(T first, T second)
+    {
+        First = first;
+        Second = second;
+    }
+
+    public T First { get; set; }
+    public T Second { get; set; }
+}
+
+interface IPair<T>
+{
+    T First { get; set; }
+    T Second { get; set; }
+}
+
+public class Contact : PdaItem
+{
+    public Contact(string name)
+        : base(name)
+    {
+    }
+}
+
+public abstract class PdaItem
+{
+    public PdaItem(string name)
+    {
+        Name = name;
+    }
+
+    public virtual string Name { get; set; }
+}