Add audit event when the "Collect usage data" setting is changed

What does this MR do and why?

Adds an audit event when the "Collect usage data" (ai_usage_data_collection_enabled) setting is changed on a group. This allows administrators to track who changed the setting and when.

Closes #595736

Implementation

The ai_usage_data_collection_enabled setting is stored in the namespace_ai_settings table (via Ai::NamespaceSetting), separate from the namespace_settings table that is already audited by NamespaceSettingChangesAuditor.

This MR adds:

1. Namespaces::AiNamespaceSettingChangesAuditor - A new auditor class following the same pattern as NamespaceSettingChangesAuditor, but for the namespace_ai_settings table. Currently audits ai_usage_data_collection_enabled, but is designed to be extensible for future Ai::NamespaceSetting columns.

2. Integration into GroupChangesAuditor - The new auditor is called from GroupChangesAuditor#execute, alongside the existing NamespaceSettingChangesAuditor and NamespaceDetailChangesAuditor calls.

3. Audit event type YAML - ai_usage_data_collection_enabled_updated event type definition.

4. Tests - RSpec tests for the new auditor.

References

• Issue: #595736
• Related feature: #587976 (closed)

Screenshots or screen recordings

No UI changes.

Before	After
No audit event logged when "Collect usage data" setting is changed	Audit event ai_usage_data_collection_enabled_updated is logged with from/to values and actor

How to set up and validate locally

1. Enable the feature in rails console or via the group settings UI
2. Navigate to a group's GitLab Duo settings and toggle the "Collect usage data" setting
3. Check the audit log for the group - you should see an ai_usage_data_collection_enabled_updated event with the old and new values

MR acceptance checklist

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