fix: copying failed if a target placeholders was missing (#8399) (#8410)

* fix: handle missing target placeholders gracefully

* fix: add regression test for copying plugins to language with missing placeholders

* fix: enhance regression test to verify handling of DoesNotExist exception during placeholder copying

Co-authored-by: Moritz Pietzschke <pietzschke@users.noreply.github.com>

Author: fsbraun