[fix] Add missing full_clean() to VPN server auto-cert creation

AbstractVpn._auto_create_cert() was saving the server certificate
without calling full_clean(), bypassing Django model validation
(PEM format, extension structure, field constraints). The client-side
equivalent in AbstractVpnClient._auto_create_cert() correctly calls
full_clean() before save(). This aligns the server-side path with the
client-side pattern.

Author: mn-ram

openwisp_controller/config/base/vpn.py

@@ -431,6 +431,7 @@ def _auto_create_cert(self):
             extensions=server_extensions,
         )
         cert = self._auto_create_cert_extra(cert)
+        cert.full_clean()
         cert.save()
         return cert