fix: fix v3 code completions

What does this merge request do and why?

fix: fix v3 code completions

Completions engine now returns a list of choices, first choice is used in response. In a follow-up we should add support for returning multiple choices.

Related to #523 (closed)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.