Behavior: Pings the server to tell it that the client is still connected. The userSessionId normally comes from the Login or AuthToken command. Optionally updates the user's screen name. Returns 404 if the session is not found.

Authorization: Requires an authenticated user (class-level [Authorize]).

Side Effects: The session's last activity timestamp is updated. The user's screen name may be updated if the sn parameter is provided.

Error Scenarios: Returns 401 if the user is not authenticated. Returns 404 if the session ID is not found or has expired.