Add PHP test coverage of layout controller

@longwave conveyed that this allowed him to iterate faster on refactoring the server side in 📌 [later phase] [PP-2] Prepare for multiple component types: prefix Component config entity IDs with `sdc` Postponed , because a Kernel test is orders of magnitude faster than an end-to-end test.

I'm not convinced yet it'll make sense to do this for all (API) controllers, but assuming that it might help iterate faster in the future again — especially while we're working on this area in 📌 Add support for Blocks as Components Active and friends, anything that helps iterating faster is more than welcome :)

📌 Update CI workflows for Drupal 11 switch Active unbroke CI. Merging in upstream, now it should pass tests…

Automatically closed - issue fixed for 2 weeks with no activity.