Add audit logging for Duo workflow session deletion

What does this MR do and why?

Add audit logging for Duo workflow session deletion

• Emit a duo_session_deleted audit event from DestroyWorkflowService after a workflow is successfully destroyed, providing an audit trail for session deletions including who initiated the deletion and which session was removed.

References

• Refs: https://gitlab.com/gitlab-org/gitlab/-/work_items/593027

Screenshots or screen recordings

Before	After
It's darker than a black hole

How to set up and validate locally

1. Open the Duo Agentic Chat panel
2. Switch to the chat history / thread list view
3. Hover/focus a past conversation row — a trash/remove icon button (Delete this chat) appears next to it, click one
4. Go to related project's audit events
5. should see Deleted Duo session event like in screenshot

MR acceptance checklist

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