refactor: add convert_to_gitlab_ci workflow prompts

What does this merge request do and why?

Part of #1072 (closed)

How to set up and validate locally

• Go to http://gdk.test:3000/admin/gitlab_duo and click on "Turn on GitLab Duo Agent Platform"
• Ensure the duo_workflow_prompt_registry feature flag is enabled at http://gdk.test:3000/rails/features/
• Ensure the duo_workflow_use_composite_identity feature flag is disabled at http://gdk.test:3000/rails/features/ (this is not production-ready yet and breaks auth)
• Tail the DWS logs and look for messages emitted by the prompt code stack, for example: gdk tail duo-workflow-service | grep "Request to LLM"
• Add a Jenkinsfile to http://gdk.test:3000/gitlab-duo/test. See an example content at https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/blob/main/docs/workflows/convert_to_gitlab_ci.md?ref_type=heads#jenkins-input
• Follow the steps at https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/blob/main/docs/workflows/convert_to_gitlab_ci.md?ref_type=heads#how-to-use to trigger the workflow
• Check that the workflow successfully creates an MR in your project to add a .gitlab-ci.yml migrated configuration

Merge request checklist

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