Fix build, address feedback, and remove Readers from trim tests

Author: captainsafia

test/Microsoft.OpenApi.Tests/PublicApi/PublicApi.approved.txt

@@ -24,7 +24,7 @@ namespace Microsoft.OpenApi.Any
         public OpenApiAnyCloneHelper() { }
         [System.Obsolete("Use native AoT-friendly generic overload of CloneFromCopyConstructor instead.")]
         public static Microsoft.OpenApi.Any.IOpenApiAny CloneFromCopyConstructor(Microsoft.OpenApi.Any.IOpenApiAny obj) { }
-        public static Microsoft.OpenApi.Any.IOpenApiAny CloneFromCopyConstructor<T>(T obj)
+        public static T CloneFromCopyConstructor<T>(T obj)
             where T : Microsoft.OpenApi.Any.IOpenApiAny { }
     }
     public class OpenApiArray : System.Collections.Generic.List<Microsoft.OpenApi.Any.IOpenApiAny>, Microsoft.OpenApi.Any.IOpenApiAny, Microsoft.OpenApi.Interfaces.IOpenApiElement, Microsoft.OpenApi.Interfaces.IOpenApiExtension
@@ -249,6 +249,7 @@ namespace Microsoft.OpenApi.Extensions
 {
     public static class EnumExtensions
     {
+        [System.Obsolete("GetAttributeOfType<T> is deprecated and will be removed in a future release.")]
         public static T GetAttributeOfType<T>(this System.Enum enumValue)
             where T : System.Attribute { }
         public static string GetDisplayName(this Microsoft.OpenApi.Models.ParameterLocation parameterLocation) { }