Route GitLabDuo MR mentions to DAP flow via messaging adapter

What does this MR do and why?

For DAP users, @GitLabDuo mentions in MR comments currently route to classic Duo Chat, which gives lower-quality responses and incorrect billing. Classic chat will be blocked for Duo Core/Pro users at 18.11

This MR introduces the assistant_flow intent path: when DuoCodeReviewChatWorker classifies a mention as assistant_flow, it delegates to AssistantFlowHandler, which triggers the DAP gitlab_duo_mention_assistant flow via the new GitlabDuoNote messaging adapter. Three conditions must hold for this path to activate: the ai_duo_mention_messaging_adapter feature flag is enabled, the namespace is in DAP mode, and a service account is configured for the code_review/v1 foundational flow. If any condition is not met, the worker falls through to classic chat unchanged.

AssistantFlowHandler resolves the catalog item + service account, builds a goal from the discussion/diff context, and calls adapter.trigger. Returns false if SA is not configured so the worker can fall back.

GitlabDuoNote adapter posts a progress note on on_request_received, then delivers or errors into the original discussion thread via deliver_result / deliver_error.

References

• Depends on: !237636 (merged) — refactor that introduces the handler class structure (this MR targets that branch)
• AIGW flow + prompt: ai-assist !5495 (merged) — classify_code_review_mention_intent prompt must return assistant_flow
• Messaging adapter infrastructure: !236207 (merged) (merged)
• Issue: https://gitlab.com/gitlab-org/gitlab/-/work_items/585237

How to set up and validate locally

Manual validation — assistant_flow path

1. Enable the feature flag: Feature.enable(:ai_duo_mention_messaging_adapter)
2. Ensure the namespace is in DAP mode and the code_review/v1 flow has a service account configured
3. Comment @GitLabDuo Can you help me understand this approach? on a diff in an MR
4. Confirm routing to AssistantFlowHandler in logs, progress note appears, and flow reply is posted in the same thread

Manual validation — fallback

1. Disable the feature flag (or use a non-DAP namespace)
2. Same @GitLabDuo comment → classic chat path: progress note + chat response

Screenshots

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