Make the eager-enumerator a sealed class (dotnet#113927)

Per [review feedback from @stephentoub](dotnet#113775 (comment)) post-merge

Author: IDisposable

src/libraries/System.DirectoryServices/src/System/DirectoryServices/SearchResultCollection.cs

@@ -215,7 +215,7 @@ void ICollection.CopyTo(Array array, int index)
         }
 
         /// <summary>Provides an enumerator implementation for the array of <see cref="SearchResult"/>.</summary>
-        private struct AlreadyReadResultsEnumerator : IEnumerator
+        private sealed class AlreadyReadResultsEnumerator : IEnumerator
         {
             private readonly IEnumerator _innerEnumerator;