[FSSDK-12670] Add comment explaining the < 2 identifiers guard

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jaeopt

optimizely/odp/odp_event_manager.py

@@ -281,6 +281,8 @@ def identify_user(self, identifiers: dict[str, str]) -> None:
         # Filter out identifiers with None or empty string values.
         valid_identifiers = {k: v for k, v in identifiers.items() if v}
 
+        # Identify requires 2+ identifiers to link (e.g., vuid + fs_user_id).
+        # A single identifier has no cross-reference value and generates unnecessary traffic.
         if len(valid_identifiers) < 2:
             self.logger.debug(
                 'ODP identify event is not dispatched (fewer than 2 valid identifiers).'