Add feature flag for Duo Workflow with Claude 4.0

What does this MR do and why?

Adding feature flag for Claude Sonnet 4

References

#545195 (closed)

#545193 (closed)

gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!2589 (merged)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. In rails console, enable duo_workflow_claude_sonnet_4 feature flag

   Feature.enable(:duo_workflow_claude_sonnet_4)

2. Make a head request to workflow endpoint curl --head -I http://gdk.test:3000/api/v4/ai/duo_workflows/workflows/945\?private_token\=$GDK_TOKEN
3. You should see duo_workflow_claude_sonnet_4 in the X-Gitlab-Enabled-Feature-Flags header

MR acceptance checklist

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