Indicate WP subject as non-writable when generated

Automatic subject generation should prevent users from
editing the subject directly. Instead it can only be updated
automatically, when other fields change in that case.

lib/api/v3/work_packages/schema/base_work_package_schema.rb

@@ -53,6 +53,7 @@ def available_custom_fields
 
           def writable?(property)
             property = property.to_s
+            return false if property == "subject" && type&.replacement_pattern_defined_for?(:subject)
 
             # Special case for milestones + date property
             property = "start_date" if property == "date" && milestone?