chore: reintroduce "Refactor Code Completions to use Prompt Registry in all cases"

What does this merge request do and why?

Reintroduces !3065 (merged) and fixes the production errors mentioned in !3065 (comment 2790750631)

The production errors were caused by routing all code completion through CodeCompletion processor which contained a bug when it could not determine the language of the file

editor_lang or resolve_lang_name(file_name) might return None, which then calls lower() on the result.

The AmazonQModel branch already handled this, but putting more code completions through AgentModel caused it to be exposed there as well

Updating the AmazonQModel completion branch to handle when the language is None should take care of the issue.

Also updates the prompt registry solution to properly handle prompt caching and session ids for Fireworks models

How to set up and validate locally

curl -X "POST" "http://localhost:5052/v2/code/completions" \
     -H 'Content-Type: application/json; charset=utf-8' \
     -d $'{
  "project_path": "gitlab-org/gitlab-shell",
  "current_file": {
    "file_name": "test.py",
    "content_above_cursor": "def is_even(n: int) ->",
    "content_below_cursor": ""
  },
  "choices_count": 2,
  "prompt_version": 1,
  "project_id": 33191677
}'

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.
• If this change requires executor implementation: verified that issues/MRs exist for both Go executor and Node executor or confirmed that changes are backward-compatible and don't break existing executor functionality.

Closes #906