Call code generation agent based on feature flag

What does this MR do and why?

• Related AI Gateway MR: feat(agents): execute code generations via agent (gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!1096 - merged)
• Related issue: Prompt Migration: collaboration issue to for Co... (#473394 - closed)

This MR sends agent_id: "code_suggestions/generations" if ai_custom_models_prompts_migration is enabled. It calls the prompt defined in AI Gateway instead of the prompt defined in Rails.