Integrate Duo Agent Platform Code Review flow with existing triggering points

Description

Integrate the new Duo Code Review flow with existing triggering points where the legacy flow is currently being called.

There are 3 triggering points today:

• When @GitLabDuo bot is assigned as a reviewer of a MR
• When user mentions @GitLabDuo asking for a review
• When creating a new MR and project configuration is set so review is asked automatically

Technical Requirements

• Create a new feature flag
• Hook into all 3 triggering points mentioned above and add a condition to trigger Duo Agent Platform flow

Acceptance Criteria

• There is a feature flag to allow alternating between new and legacy flow
• When flag is disabled, the existing flow (called now "legacy flow") is executed - this is the default
• When flag is enabled, the new Duo Code Review flow running on Duo Agent Platform is executed