[tests] Fix API regression test: replace VPN template instead of clearing openwisp#1221

The test must send a non-empty templates list (replacing the VPN template
with a generic one) so that the m2m post_add signal fires and triggers
manage_vpn_clients to delete the stale VpnClient via per-instance delete.
Sending an empty templates list hits only the post_clear path which skips
deletion for non-deactivated devices.

Co-Authored-By: mn-ram <mn-ram@users.noreply.github.com>

Author: mn-ram

openwisp_controller/config/tests/test_api.py

@@ -520,14 +520,15 @@ def test_device_patch_with_templates_of_different_org(self):
         )
 
     def test_device_patch_vpn_template_removal_triggers_post_delete(self):
-        """Regression test for #1221: removing a VPN template via PATCH API
-        must trigger VpnClient.post_delete so peer cache is invalidated
-        and the certificate is revoked."""
+        """Regression test for #1221: replacing a VPN template with a generic
+        one via PATCH API must trigger VpnClient.post_delete so peer cache is
+        invalidated and the certificate is revoked."""
         org = self._get_org()
         vpn = self._create_vpn(organization=org)
         t_vpn = self._create_template(
             name="vpn-test", type="vpn", vpn=vpn, auto_cert=True, organization=org
         )
+        t_generic = self._create_template(name="generic-test", organization=org)
         device = self._create_device(organization=org)
         config = self._create_config(device=device)
         config.templates.add(t_vpn)
@@ -536,8 +537,8 @@ def test_device_patch_vpn_template_removal_triggers_post_delete(self):
         cert_pk = vpnclient.cert.pk
 
         path = reverse("config_api:device_detail", args=[device.pk])
-        # PATCH with empty templates list to remove the VPN template
-        data = {"config": {"templates": []}}
+        # PATCH replacing VPN template with a generic template
+        data = {"config": {"templates": [str(t_generic.pk)]}}
         with patch.object(Vpn, "_invalidate_peer_cache") as mock_invalidate:
             response = self.client.patch(path, data, content_type="application/json")
             mock_invalidate.assert_called_once()