16770 work package api patch for subject

[``* `#16770` Work package API PATCH for subject``](https://community.openproject.org/work_packages/16770)

This PR applies the specified behavior outlined in #2056.
I interpreted the error identifier as part of the representation and removed them from the actual error objects intentionally (see https://github.com/opf/openproject/commit/6dc02bca0f8b2d000c01a3cc697f94fbbd00389e). But I can change that of course.