Fix event payload to handle holdout dict variations

CRITICAL FIX: Holdout impression events had empty variation_key and
variation_id in event payloads because event_factory only checked for
Variation entities, not dict variations.

Root Cause:
- event_factory.py lines 125-127 only handled entities.Variation type
- For holdouts, event.variation is a dict (VariationDict), not Variation entity
- isinstance(event.variation, entities.Variation) returned False for holdouts
- variation_id and variation_key stayed as empty strings ('')
- Event payload sent with empty variation_key and variation_id fields

The Fix:
- Added elif clause to handle dict variations
- Extract 'id' and 'key' from dict using .get() method
- Preserves existing behavior for Variation entities
- Now handles both Variation entities and VariationDict formats

Code Changes (event_factory.py lines 125-131):
```python
if isinstance(event.variation, entities.Variation):
    variation_id = event.variation.id
    variation_key = event.variation.key
elif isinstance(event.variation, dict):
    # Handle holdout variations (dict format)
    variation_id = event.variation.get('id', '')
    variation_key = event.variation.get('key', '')
```

Impact:
- Holdout impression events now include correct variation_key and variation_id
- Event metadata properly populated for holdout decisions
- Fixes fullstack compatibility suite decide_holdouts.feature validation
- Fixes decide_all_api_with_holdouts.feature validation
- All 54 tests passing (35 holdout + 19 event_factory)
- Backward compatible with existing Variation entity handling

Aligned with Swift SDK event payload structure.

Author: muzahidul-opti

optimizely/event/event_factory.py

@@ -125,6 +125,10 @@ def _create_visitor(cls, event: Optional[user_event.UserEvent], logger: Logger)
             if isinstance(event.variation, entities.Variation):
                 variation_id = event.variation.id
                 variation_key = event.variation.key
+            elif isinstance(event.variation, dict):
+                # Handle holdout variations (dict format)
+                variation_id = event.variation.get('id', '')
+                variation_key = event.variation.get('key', '')
 
             if event.experiment:
                 experiment_layerId = event.experiment.layerId