fix: printer agent switching on preset change

src/slic3r/GUI/DeviceManager.hpp

@@ -305,6 +305,9 @@ public:
     /** Whether this printer supports virtual trays (external/manual filament loading).
      *  When true, vt_slot data is used by build_filament_ams_list() to include external filaments. */
     bool  ams_support_virtual_tray { true };
+    // Filament entries supplied by a printer agent (for example, toolchanger tools)
+    // remain valid even when a regular status message has no vir_slot field.
+    bool  agent_virtual_tray { false };
     time_t ams_user_setting_start = 0;
     time_t ams_switch_filament_start = 0;
     AmsStatusMain ams_status_main;