Classify "GitLabDuo" MR mention intent to route code review requests

What does this MR do and why?

When a user mentions @GitLabDuo in an MR comment, this MR classifies the intent of the message and routes code_review_request intents to the existing code review flow. All other intents (duo_developer, agentic_chat, or unknown) continue to use the existing Duo Chat flow unchanged.

This is behind the classify_code_review_mention_intent (gitlab_com_derisk) feature flag.

AIGW counterpart is already merged: feat: add classify_code_review_mention_intent p... (gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!5198 - merged)

How it works

1. In DuoCodeReviewChatWorker#perform, before starting the chat flow, we call code_review_intent?(note)
2. This strips @mentions from the note, then calls the ClassifyCodeReviewMentionIntent AI Gateway completion with the message text
3. If the intent is code_review_request:
   • If a review is already in progress → reply with an in-progress message and return
   • If @GitLabDuo is already a reviewer → call RequestReviewService to re-request
   • Otherwise → call UpdateReviewersService to assign @GitLabDuo as reviewer
4. Both services route to DAP or classic via request_duo_code_review in the EE override — no manual branching needed
5. If classification fails for any reason → fall back to the chat flow (existing behaviour preserved)

Screenshots

Intent: code_review_request (GitLabDuo already added as a reviewer)	Intent: code_review_request (GitLabDuo not already added as a reviewer yet)

Intent: duo_developer (just uses the current classic chat for now)	Intent: agentic_chat (just uses the current classic chat for now)

Intent: code_review_request (multiple reviews)

How to set up and validate locally

1. Enable the feature flag:

   Feature.enable(:classify_code_review_mention_intent)

2. Test scenarios on any MR:

   Scenario	Comment	Expected Result
   Code review request	@GitLabDuo please review this MR	Adds GitLabDuo as reviewer (or re-requests if already added)
   Review already in progress	Same as above while review is running	Replies "I'm already reviewing this merge request..."
   Duo Developer intent	@GitLabDuo fix the failing test	Falls back to chat flow
   Agentic chat intent	@GitLabDuo what does this code do?	Falls back to chat flow

3. Disable the flag to verify fallback:

   Feature.disable(:classify_code_review_mention_intent)

   All @GitLabDuo mentions should use the existing chat flow.

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.

Related to:

• https://gitlab.com/gitlab-org/gitlab/-/work_items/595294+
• https://gitlab.com/groups/gitlab-org/-/work_items/21555+