Add audit event for enabled_foundational_flows changes

What does this MR do and why?

Add audit event for enabled_foundational_flows changes

Add enabled_foundational_flows_updated audit event to track when the selected foundational flows are changed on a group. Uses a custom auditor that captures previous and new flow references, emitting a human-readable audit message.

• Create audit event type YAML definition
• Add EnabledFoundationalFlowsAuditor class
• Wire auditor into Groups::UpdateService
• Add spec coverage for the new auditor

References

• Refs: https://gitlab.com/gitlab-org/gitlab/-/work_items/593019

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Start a GDK instance with EE license features enabled and foundational flows catalog seeded
2. As a group owner, navigate to a group's settings -> GitLab Duo, and change the enabled foundational flows selection
3. Check the audit events log for the group
4. Verify an audit event was created with:
   • Event type: enabled_foundational_flows_updated
   • Message like: Changed enabled foundational flows from none to chat/v1, code_review/v1
5. Verify that submitting the same set of flows again does NOT create a duplicate audit event

MR acceptance checklist

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