Investigate how to trigger a Duo Agent Platform flow for code review

What is the goal of this spike?

Investigate how to trigger the Duo Agent Platform Code Review flow when a review from GitLab Duo Bot is requested.

Scope/MVC

Currently, MR authors can request a review from GitLab Duo when:

• a MR is opened when project configuration is set to request a review automatically
• GitLab Duo Bot is assigned as a reviewer of a MR
• the MR author explicitly requests a review from GitLab Duo Bot

These 3 events will kick off Duo Code Review and they are entry points for the new Duo Agent Platform Code Review flow.

Acceptance criteria

At the end of the spike, we should be able to trigger a flow (see Investigate how to create a Duo Agent Platform ... (#565926 - closed)) whenever one of the events above are triggered.