Maintain ordering of properties as specified in fixedFieldMap by doing a union of fixed field map keys and property names in the map node

Author: MaggieKimani1

src/Microsoft.OpenApi.Readers/V2/OpenApiV2Deserializer.cs

@@ -29,10 +29,11 @@ private static void ParseMap<T>(
                 return;
             }
 
-            foreach (var propertyNode in mapNode)
+            var allFields = fixedFieldMap.Keys.Union(mapNode.Select(x => x.Name));
+            foreach (var propertyNode in allFields)
             {
-                propertyNode.ParseField(domainObject, fixedFieldMap, patternFieldMap);
-                requiredFields?.Remove(propertyNode.Name);
+                mapNode[propertyNode]?.ParseField(domainObject, fixedFieldMap, patternFieldMap);
+                requiredFields?.Remove(propertyNode);
             }
         }