Fix diff mode attribute access

- Use self.module._diff instead of self.module.diff
- _diff is the correct internal attribute in Ansible's AnsibleModule

plugins/modules/nsupdate_zone.py

@@ -833,7 +833,7 @@ class DNSZoneManager:
             result['changes']['updates'] = len(changes['updates'])
             
             # Add diff output if enabled
-            if self.module.diff:
+            if self.module._diff:
                 result['diff'] = self._format_diff(changes)
             
             # Step 8: Apply changes (unless check mode)