Extend config schema for custom_elements.entity_ce_display

This more urgent now because the lack of config schema makes PHPUnit testing for lupus_decoupled fail.

I don't know this module's code yet, but it was easy enough to see (from searching or "config") that the 'custom_elements.settings' schema contains just one setting. So

• if this is the only thing that needed to be checked, this issue can be marked 'fixed' after merging
• if not / if possibly there are other config elements (i.e. entities) to be described, this should stay open after merging.

Please either

• merge the 3362630-config-schema-v2, ignore the other one, and merge that into 3.x. Downside: will get a (trivial) merge conflict.
• or merge the 3362630-config-schema-v2 into 8.x-2.x and 3362630-config-schema into 3.x. Downside: branches will diverge.

Code changes look good and we can merge them.
Except I'm not sure if that's what this ticket is about. I think this ticket also expects the checks of custom_elements.entity_ce_display.*.*.* schema.

So I propose we merge both PRs to help 🐛 Fix PHPCS and maybe other errors Needs review . Then keep this ticket open to check the schema of entity_ce_display.

I guess back to active because that was the status when I barged in here.

Yes, this misses the config entity type, so seems not complete.

Running unit test with strict config schema gives the following error:

Drupal\Tests\custom_elements\Functional\CustomElementsRenderMarkupVue3Test::testNodeRendering
Drupal\Core\Config\Schema\SchemaIncompleteException: Schema errors for media.type.image with the following errors: media.type.image:source_configuration.gather_exif missing schema

Tried to fix it quickly but I couldn't do, it needs more investigation and deeper understanding of schema structure.

that seems to be an unrelated issue and could be some issue with the default-config placed into the thunder module also.
Maybe we should have a dedicated config-schema check test-case which runs without the thunder module, the default-config issues of the thunder sub-module are really not important here.