Update to seat `created_at` timestamp in the Copilot user management REST API [Public Preview]

Based on customer feedback, we have updated how the created_at timestamp works in the Copilot seat details portion of responses from the following REST API endpoints:

• /organization/{org}/billing/copilot/seats
• /enterprises/{enterprise}/billing/copilot/seats
• /organization/{org}/members/{username}/copilot

The created_at timestamp now shows when a user received Copilot access, rather than when their team, enterprise team, or organization was granted access. This matches the timestamp of the seat’s corresponding seat_added event in the Audit Log.