Expose `CODE_ASSIST_API_VERSION` similar to already exposed `CODE_ASSIST_ENDPOINT`

[ffeldhaus]

What would you like to be added?

For testing purposes it is important to be able to override not only the already exposed CODE_ASSIST_ENDPOINT but also the CODE_ASSIST_API_VERSION.

Why is this needed?

This is required to support testing scenarios with different endpoints and versions.

Additional context

No response