Change return type from Stream to MemoryStream for improved performance

Author: MaggieKimani1

src/Microsoft.OpenApi.Hidi/OpenApiService.cs

@@ -307,7 +307,7 @@ private static XslCompiledTransform GetFilterTransform()
             return transform;
         }
 
-        private static Stream ApplyFilterToCsdl(Stream csdlStream, string entitySetOrSingleton, XslCompiledTransform transform)
+        private static MemoryStream ApplyFilterToCsdl(Stream csdlStream, string entitySetOrSingleton, XslCompiledTransform transform)
         {
             using StreamReader inputReader = new(csdlStream, leaveOpen: true);
             using var inputXmlReader = XmlReader.Create(inputReader);