Fix AI audit events storage group setting persistence

What does this MR do and why?

The "Store AI audit events" checkbox on the group GitLab Duo settings page (/-/settings/gitlab_duo/configuration) never persisted. The frontend sent the value nested as namespace_settings_attributes: { ai_audit_events_storage_enabled } in PUT /api/v4/groups/:id, but that param is not declared in the Grape API, so declared_params silently dropped it. The request returned 200 with the success alert while the setting was never applied.

This MR:

• Declares ai_audit_events_storage_enabled and lock_ai_audit_events_storage_enabled as top-level params on PUT /groups/:id, following the existing convention for namespace settings (like duo_features_enabled / lock_duo_features_enabled). Groups::UpdateService#handle_namespace_settings already handles them because both attributes are present in NamespaceSetting.allowed_namespace_settings_params.
• Updates the frontend to send the flat param instead of the nested hash.

The GraphQL groupUpdate mutation already accepts both arguments; only the REST path was broken.

Video

How to set up and validate locally

1. Enable the agent_artifacts_page feature flag (default enabled).
2. Go to Group → Settings → GitLab Duo → Configuration.
3. Toggle Store AI audit events and save.
4. Reload the page — the checkbox state now persists.

References

• Feature issue: https://gitlab.com/gitlab-org/gitlab/-/issues/592473