Fix auth bypass external flag is not respected

What does this merge request do and why?

This MR fixes that AIGW_AUTH__BYPASS_EXTERNAL=true flag stopped taking an effect after fix: use a custom self issued scope for user JWT (!872 - merged). This issue is currently failing CI jobs in GitLab-Rails https://gitlab.slack.com/archives/C051K31F30R/p1717479004054009.

The root cause is that, no places import ai_gateway.auth.authentication monkey patch, which includes if conn.user.is_debug is True: flag to respect the AIGW_AUTH__BYPASS_EXTERNAL flag. It turned out the patch is silently overriding starlette.authentication.requires, so we didn't notice this issue for while.

We're fully migrating to fine-grained auth logic, so that we can remove the patch.

How to set up and validate locally

1. Set AIGW_AUTH__BYPASS_EXTERNAL to True.

Test request and confirm that the request succeeds:

curl -X 'POST'   'http://localhost:5052/v1/search/gitlab-docs'   -H 'accept: application/json'   -H 'Content-Type: application/json'   -d '{
  "type": "string",
  "metadata": {
    "source": "string",
    "version": "string"
  },
  "payload": {
    "query": "string"
  }
}'

Merge request checklist

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