Add code review endpoint for Duo Agent flow

What does this MR do and why?

This MR adds a new endpoint /api/v4/ai/duo_workflows/code_review/add_comments which will be used by AIGW flow to post review comments from Duo Code Review.

The current flow is being ported to Duo Agent Platform and we need to refactor part of the existing code to support that migration.

This endpoint is not in use yet and a follow-up MR will introduce a feature flag to control whether to use the new Duo Workflow or the legacy path.

Epic Integrate Duo Code Review into Duo Agent Platform (&18645 - closed)

Related to #570309 (closed)

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