fix bogus errors in contracts

- AR tried to read properties that are undefined on the contract (:error_read_only)
- now that we actually use Contract properties the copy-validations stuff started to do things, however:
    - we already went another path to simultaneously validate contract + model
    - validation was apparently broken, e.g. though the subject was not empty, there were subject :blank errors

lib/api/contracts/model_contract.rb

@@ -52,6 +52,17 @@ module API
         end
       end
 
+      # we want to add a validation error whenever someone sets a property that we don't know.
+      # However AR will cleverly try to resolve the value for errorneous properties. Thus we need
+      # to hook into this method and return nil for unknown properties to avoid NoMethod errors...
+      def read_attribute_for_validation(attribute)
+        if respond_to? attribute
+          send attribute
+        end
+
+        nil
+      end
+
       def writable_attributes
         collect_ancestor_attributes(:writable_attributes)
       end

lib/api/v3/work_packages/base_contract.rb

@@ -97,9 +97,6 @@ module API
           @can = WorkPackagePolicy.new(user)
         end
 
-        extend Reform::Form::ActiveModel::ModelValidations
-        copy_validations_from WorkPackage
-
         private
 
         def validate_people_visible(attribute, id_attribute, list)