fix: Move call to `AiRelatedSettingsChangedEvent`

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What does this MR do and why?

This MR fixes a potential issue with the AiRelatedSettingsChangedEvent being published prematurely. Previously, the event was being emitted from NamespaceSettings::AssignAttributesService which only assigns attributes but doesn't persist them. This could lead to events being published for changes that weren't actually saved to the database.

Changes made:

1. Moved event publishing from NamespaceSettings::AssignAttributesService to Groups::UpdateService's after_update callback
2. Updated the logic to check for actual persisted changes using saved_change_to_attribute?
3. Added comprehensive specs to verify event publishing behavior

This ensures that events are only published when:

• The update is successful
• There are actual persisted changes to AI-related settings
• The namespace settings exist

References

• Issue #444224 (closed): Move AiRelatedSettingsChangedEvent to after_update

How to set up and validate locally

1. Start GDK:

   gdk start

2. In rails console, set up a test group:

   # Create organization
   organization = Organizations::Organization.create!(
     name: 'Test Organization',
     path: 'test-organization'
   )
   
   # Create group
   user = User.first
   group = Group.create!(
     name: 'test-group',
     path: 'test-group',
     visibility_level: Gitlab::VisibilityLevel::PRIVATE,
     organization: organization
   )
   group.add_owner(user)

3. Test the AI settings change:

   # Update settings
   service = Groups::UpdateService.new(group, user, { experiment_features_enabled: true })
   result = service.execute
   
   # Verify change
   group.reload.namespace_settings.experiment_features_enabled # Should be true

4. Monitor logs to verify event publishing:

   gdk tail | grep AiRelatedSettingsChangedEvent

MR acceptance checklist

• The change is isolated to the event publishing logic
• Added comprehensive specs for success and failure cases
• No changes to database schema
• No changes to API endpoints
• Event is only published after successful updates