feat: Update participant_id assignment in CongoSameDiff class (#1004)

The participant_id assignment in the CongoSameDiff class has been updated to use the session's participant ID instead of generating using the participant_id_url property as a base for the pattern index.

backend/experiment/rules/congosamediff.py

@@ -114,7 +114,7 @@ def next_round(self, session: Session):
         variants_amount = real_trial_variants.count()
 
         # get the participant's group variant based on the participant's id # else default to random number between 1 and variants_amount
-        participant_id = int(session.participant.participant_id_url) if session.participant.participant_id_url else random.randint(1, variants_amount * 2)
+        participant_id = session.participant.id
         participant_group_variant = self.get_participant_group_variant(
             participant_id,
             group_number,