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/libslic3r/AppConfig.cpp

@@ -856,6 +856,10 @@ std::string AppConfig::load()
                         local_machine.dev_ip = p["dev_ip"].get<std::string>();
                     if (p.contains("printer_type"))
                         local_machine.printer_type = p["printer_type"].get<std::string>();
+                    if (p.contains("printer_agent_id"))
+                        local_machine.printer_agent_id = p["printer_agent_id"].get<std::string>();
+                    if (p.contains("access_code"))
+                        local_machine.access_code = p["access_code"].get<std::string>();
                     m_local_machines[local_machine.dev_id] = local_machine;
                 }
             } else {
@@ -1068,6 +1072,8 @@ void AppConfig::save()
         m_json["dev_name"]         = local_machine.second.dev_name;
         m_json["dev_ip"]           = local_machine.second.dev_ip;
         m_json["printer_type"]     = local_machine.second.printer_type;
+        m_json["printer_agent_id"] = local_machine.second.printer_agent_id;
+        m_json["access_code"]      = local_machine.second.access_code;
 
         j["local_machines"][local_machine.first] = m_json;
     }