server/control: deduplicate close-proxy logic and UserInfo construction (#5218)

Extract closeProxy() helper to eliminate duplicated 4-step cleanup
sequence (Close, PxyManager.Del, metrics, plugin notify) between
worker() and CloseProxy().

Extract loginUserInfo() helper to eliminate 4 repeated plugin.UserInfo
constructions using LoginMsg fields.

Optimize worker() to snapshot and clear the proxies map under lock,
then perform cleanup outside the lock to reduce lock hold time.