Ensure the -best model points to the appropriate language model

[stephan-buckmaster]

Added new method LanguageModel#effective_api_name. When the api_name of a LanguageModel ends in -best this method looks up the language model of its APIService that has the best flag set.

1. I don't think the Export class needs to resolve the (new) effective_api_name.
2. The Export functionality seems the only where the Assistant code references api_name. So no need to change the Assistant model code.
3. Added comment about the language model Test link (such a nice feature) that it should also take into account the API Service (File app/controllers/settings/language_models_controller.rb)

Fixes issue #592

[krschacht]

I reviewed this PR but I'm torn with regards to the implementation. The challenge with replacing all the api_name calls with effective_api_name is that we're not fully replacing this "virtual" language_model with the resulting language_model that we want it to act like. So code that references any other attributes of this language_model will potentially get wrong info (e.g. cost per token and flags like supports_image).

My first thought was: can we make the call to .language_model somehow recognize that it's a special "virtual/pointer/best" model and actually return the model that we want it to act like. But since language_model is an association, I couldn't figure out an appropriate way to hook in there.

So then I was thinking that we could let language_model be the virtual one, but within it, any call to any attribute would do a lookup of the actual best: true model and return that attribute instead. But @attributes is an ivar, not a method, and I couldn't figure out an elegant way to tap into that and intercept any attempt to reference an attribute.

I see how you ended up where you did. I can't think of a better solution. But I worry that this solution will be too brittle. There may already be subtle bugs in the code for places where we aren't using the correct language_model attribute. I don't have a good idea for how to resolve... I'm stuck... :)

[stephan-buckmaster]

I don't know, this seems to get very general. How often does the "best" of today change features compared to the "best" of yesterday?