[client] add one seconde to valid_until to be stix compliant

Author: SarahBocognano

pycti/utils/opencti_stix2.py

@@ -1507,7 +1507,7 @@ def generate_export(self, entity: Dict, no_custom_attributes: bool = False) -> D
             and "valid_until" in entity
             and entity["valid_from"] == entity["valid_until"]
         ):
-            del entity["valid_from"]
+            entity['valid_until'] = entity['valid_until'] + datetime.timedelta(seconds=1)
 
         # Flatten
         if "tasks" in entity: