fix: close connection on worker pool overflow

When the worker pool rejected a connection (ErrPoolOverload), the
accepted net.Conn was never closed — leaking a file descriptor and
TCP socket per rejected connection. Under sustained traffic spikes this
compounds the problem: leaked descriptors reduce the capacity for new
dials (including to the fronting domain), accelerating the failure
cascade described in #378.

mtglib/proxy.go

 		case errors.Is(err, ants.ErrPoolClosed):
 			return nil
 		case errors.Is(err, ants.ErrPoolOverload):
+			conn.Close() //nolint: errcheck
 			logger.Info("connection was concurrency limited")
 			p.eventStream.Send(p.ctx, NewEventConcurrencyLimited())
 		}