Fix holdout variation lookup in impression event creation

CRITICAL FIX: Holdout impression events were sending empty variation_key
and variation_id in event payloads, causing fullstack compatibility suite
failures.

Root Cause:
- user_event_factory.create_impression_event was using get_flag_variation()
  for all events when both flag_key and variation_id were present
- get_flag_variation() only searches flag_variations_map (experiments/rollouts)
- Holdout variations aren't in flag_variations_map, so lookup returned None
- This caused impression events to have empty variation_key and variation_id

The Fix:
- Added rule_type check: exclude HOLDOUT from get_flag_variation() path
- For holdouts: use get_variation_from_id_by_experiment_id() instead
- This works because holdout variations are properly mapped in
  variation_id_map_by_experiment_id and variation_key_map_by_experiment_id

Impact:
- Holdout impression events now include correct variation_key and variation_id
- Event metadata properly populated: rule_key, rule_type: holdout, variation_key
- Fixes fullstack compatibility suite decide_holdouts.feature event validation
- All 237 tests passing (194 optimizely + 8 event_factory + 35 holdout)

Aligned with Swift SDK event creation behavior.

Author: muzahidul-opti

optimizely/event/user_event_factory.py

@@ -69,11 +69,13 @@ def create_impression_event(
         if activated_experiment:
             experiment_id = activated_experiment.id
 
-        if variation_id and flag_key:
+        if variation_id and flag_key and rule_type != enums.DecisionSources.HOLDOUT:
             # need this condition when we send events involving forced decisions
             # (F-to-D or E-to-D with any ruleKey/variationKey combinations)
+            # But NOT for holdouts - they use experiment_id lookup instead
             variation = project_config.get_flag_variation(flag_key, 'id', variation_id)
         elif variation_id and experiment_id:
+            # For holdouts, experiments, and rollouts - lookup by experiment/holdout ID
             variation = project_config.get_variation_from_id_by_experiment_id(experiment_id, variation_id)
 
         event_context = user_event.EventContext(