Route Duo Code Review to the advanced flow behind a feature flag

What does this MR do and why?

Routes Duo Code Review to the new advanced flow behind a default-off feature flag, without changing how review output is parsed.

• Adds the beta flag duo_code_review_advanced_flow (user actor, off by default).
• In the foundational flow resolver, routes code_review/v1 to advanced_code_review/v1 version 1.0.0 when the flag is enabled for the requesting user.
• This check is placed before the existing duo_code_review_previous_discussions branch, so it takes precedence over it.

With the flag on, the advanced flow still delivers its review through the existing XML review_output transport, which the current parser already handles. This MR alone is functional and safe to merge and deploy: it only changes routing, not parsing.

The structured findings (JSON) transport is a stacked follow-up: !253794 (merged). It targets this MR's branch and depends on the flag defined here.

No changelog: behind a default-off flag.

Screenshots or screen recordings

N/A, backend-only change.

How to set up and validate locally

1. Enable the flag in a Rails console: Feature.enable(:duo_code_review_advanced_flow, User.find_by_username('your-username'))
2. Request a Duo Code Review on a merge request and confirm the advanced flow runs.
3. Run the spec: bundle exec rspec ee/spec/lib/ai/duo_workflows/foundational_flow_start_params_resolver_spec.rb

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.

References

• Tracking work item: #627566
• Parent epic: gitlab-org#21524
• Benchmark results: https://gitlab.com/gitlab-org/gitlab/-/work_items/618672#note_3770841977
• Companion ai-assist MR: gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!6773 (merged)
• Rollout issue: #627796
• Stacked follow-up: !253794 (merged)