Let sub-modules inherit the model to use from AI Core

This is actually part of the logic of third party vendors I would say. It is for instance implemented in AI Image Alt Text module, as you can see here: https://git.drupalcode.org/project/ai_image_alt_text/-/blame/1.0.x/src/P...

If we should do this globally in the form helper, it could lead to bad behvaiours, since you can pick which parts you use from the the form helper. It could in theory be done, but I'm not sure that is the best solution?

I've built this in ai_content an ai_ckeditor.
I've tested with "default" and one specific model in all cases but it would be great if someone just tested/verified this.

I will also add this functionality to ai_translate in this Issue:
https://www.drupal.org/project/ai/issues/3473232 ✨ [ai_translate] more flexibility Needs review

I was originally looking at this as if AI Core was skipping making you select a model when I read the title so at first had the same worry as Marcus. But looking at your code, you are wanting to allow sub-module to inherit from AI Core. Updated title and issue summary to make that a bit more clear.

I'm in favour of sub-modules inheriting from AI Core, I can see it causing problems for users if not as they may expect e.g. selecting an improved model to result in improved results everywhere only to be surprised to eventually find that some corner of their CMS config is using an old model.

Note I also have 📌 Remove default model selection, make the user choose (or provide a sensible default) Needs review which I think helps for Marcus' concern. Ie, force the user to choose the model in AI Core (which could eventually accept some advice from the third party vendor if they expose this, but I can't imagine they well: eg, openai defaults to 4o mini for free users in their UI, then 4o for paid plans so there is no real 'default').

Added an error if nothing is set either in ai settings or ai_content settings.

Removed those too.

I also rebased and fixed some merge conflicts with 1.0.x.
(and re-tested).

Thanks wouters_f! Getting merged!