Better test-coverage for layout builder support

created a quick test, with the help of claude code.

Note: ran into an issue that return docs of getSlot() is off, so fixed it on the way.

@fago, I approved the PR, although the comments could be improved. I added some comments to the MR.

I added comments to your MR at gitlab, not sure why they are not reflected here. please check https://git.drupalcode.org/project/custom_elements/-/merge_requests/125

passed-tr, I also tested it locally, works, pipeline is passing