feat: printer agent isolation across devices (#15147)

# Description

Printers discovered/bound under one printer agent (e.g. built-in BBL)
were leaking into another, independent agent's "My Device"/"Other
Device" lists and inheriting its saved access code, since neither the
device list nor bind state was ever scoped by which agent found them.

- Add printer_agent_id to MachineObject/BBLocalMachine, stamped at
  discovery/bind time; filter get_my_machine_list(),
  get_my_cloud_machine_list(), and update_other_devices() by it.
- clear_other_devices() now drops entries stamped by the outgoing
  agent on swap, so the incoming agent's own discovery re-inserts and
  re-stamps them fresh instead of leaving them stale-tagged forever.
- Scope access_code by (dev_id, printer_agent_id) on BBLocalMachine
  (LAN only since cloud's userMachineList is always refreshed live from
  the account API, so it isn't at risk the same way), with a
  BBL-only legacy fallback to the old flat access_code/user_access_code
  keys so existing bindings keep working.

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src/slic3r/GUI/SelectMachine.cpp

@@ -3912,7 +3912,7 @@ _collect_sorted_machines(Slic3r::DeviceManager* dev_manager,
     };
 
     // collect from user machine list
-    const auto& user_machine_list = dev_manager->get_my_machine_list();// user machine list
+    const auto& user_machine_list = dev_manager->get_my_machine_list(dev_manager->get_current_printer_agent_id());// user machine list
     for (const auto& elem : user_machine_list)
     {
         MachineObject* mobj = elem.second;