Added validation to ReadFragment

Author: darrelmiller

src/Microsoft.OpenApi.Readers/OpenApiStreamReader.cs

@@ -144,6 +144,16 @@ public T ReadFragment<T>(Stream input, OpenApiSpecVersion version, out OpenApiDi
                 diagnostic.Errors.Add(new OpenApiError(ex));
             }
 
+            // Validate the element
+            if (_settings.RuleSet != null && _settings.RuleSet.Rules.Count > 0)
+            {
+                var errors = element.Validate(_settings.RuleSet);
+                foreach (var item in errors)
+                {
+                    diagnostic.Errors.Add(item);
+                }
+            }
+
             return (T)element;
         }