Add audit logging for Ai::NamespaceSetting columns

What does this MR do and why?

Add audit logging for Ai::NamespaceSetting columns

• Extend AiNamespaceSettingChangesAuditor to cover three additional DAP-specific columns on Ai::NamespaceSetting that were previously unaudited:
  • duo_agent_platform_enabled
  • duo_workflow_mcp_enabled
  • prompt_injection_protection_level

References

• Refs: https://gitlab.com/gitlab-org/gitlab/-/work_items/593025

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