Migrate glab_ask_git_command's prompt to the AIGW

What does this MR do and why?

Part of #500895 (closed)

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Before	After

How to set up and validate locally

• Make sure your GDK has https configured (glab forces https)
• Enable FF prompt_migration_glab_ask_git_command
• Checkout the branch from gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!1523 (merged) in your AI Gateway and restart it
• GITLAB_TOKEN=$GITLAB_LOCAL_TOKEN GITLAB_HOST=gdk.test:3443 glab duo ask "apply a single commit"