[pd] daemon service: reconnect to dbus on dbus reset (openthread#2605)

This commit addresses a potential issue where the pd daemon loses its
dbus connection when original dbus is reset. The daemon now monitors
the io.openthread.BorderRouter.wpan0 dbus name and reconnects if the
name owner changes, indicating a likely otbr-agent reload. This
ensures the daemon continues to function correctly after dbus is reset
by otbr-agent.

script/reference-device/dhcp6_pd_daemon.py

@@ -100,14 +100,13 @@ def connect_to_signal():
             properties_changed_handler,
             dbus_interface=properties_dbus_iface.dbus_interface)
         logging.info("Connected to D-Bus signal.")
-        return dbus_obj
     except dbus.DBusException as e:
         logging.error(f"Error connecting to D-Bus: {e}")
-        return None
 
 
-def check_and_reconnect(dbus_obj):
-    if dbus_obj is None:
+def handle_name_owner_changed(new_owner):
+    if new_owner:
+        logging.info(f"New D-Bus owner({new_owner}) assigned, connecting...")
         connect_to_signal()
 
 
@@ -130,9 +129,8 @@ def main():
 
     loop = GLib.MainLoop()
 
-    thread_dbus_obj = connect_to_signal()
-
-    GLib.timeout_add_seconds(5, check_and_reconnect, thread_dbus_obj)
+    bus.watch_name_owner(bus_name='io.openthread.BorderRouter.wpan0',
+                         callback=handle_name_owner_changed)
 
     loop.run()