refactor: separate session responsibilities and inject dependencies

internal/port/port.go

@@ -75,7 +75,7 @@ func (pm *PortManager) SetPortStatus(port uint16, assigned bool) error {
 	defer pm.mu.Unlock()
 
 	if _, exists := pm.ports[port]; !exists {
-		return fmt.Errorf("port %d is not in the allowed range", port)
+		return fmt.Errorf("port %d already in use", port)
 	}
 	pm.ports[port] = assigned
 	return nil