Use Claude 3 Sonnet for Duo Chat Zero Shot

What does this MR do and why?

• This is behind a feature flag for testing
• Uses request body format needed for messages API, which is now supported by the AI Gateway: gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!668 (merged)
• #444629 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

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Before	After

How to set up and validate locally

1. In rails console enable the feature flag fully

   Feature.enable(:ai_claude_3_sonnet)

2. Set up the AI Gateway locally
3. Ask Duo Chat questions in your GDK

ee/config/feature_flags/gitlab_com_derisk/ai_claude_3_sonnet.yml

 ---
 name: ai_claude_3_sonnet
 feature_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/444629
-introduced_by_url:
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/148223
 rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/446302
 milestone: '16.11'
 group: group::duo chat