[fix] Guard missing skip_push_update_on_save in register view

DeviceRegisterView._update_device_name calls
``device.skip_push_update_on_save()`` whenever an agent re-registers
with a hostname different from the MAC address. While the method is
defined on AbstractDevice, subclasses or older installations that
lack it would crash with AttributeError on every such re-registration
(HTTP 500).

This breaks the consistent_registration / factory-reset workflow:

1. A router does a factory reset and loses its local UUID/key.
2. The agent computes a consistent key and POSTs to /register/.
3. OpenWISP finds the existing device by key.
4. The view enters _update_device_name (hostname differs from MAC).
5. If skip_push_update_on_save is missing, AttributeError → HTTP 500.
6. The agent sees no X-Openwisp-Controller header and aborts.
7. After 6 retries procd kills the agent.

Wrap the call in ``getattr`` so the registration path succeeds whether
the helper method exists or not. The guard is forward-compatible.

Closes #1331

Author: MichaelUray

openwisp_controller/config/controller/views.py

@@ -475,7 +475,13 @@ def _update_device_name(self, request, device):
         normalized_name = name.replace(":", "").replace("-", "").lower()
         if normalized_name != normalized_mac:
             device.name = name
-            device.skip_push_update_on_save()
+            # NOTE: ``device.skip_push_update_on_save()`` is defined on
+            # AbstractDevice, but we guard the call defensively so that
+            # subclasses or older installations that may lack the method
+            # do not crash during factory-reset re-registration.
+            skip = getattr(device, "skip_push_update_on_save", None)
+            if callable(skip):
+                skip()
 
 
 class GetVpnView(SingleObjectMixin, View):