Fix: Always evaluate holdouts for every feature (align with Swift SDK)

This commit fixes a critical architectural difference where Python
conditionally entered the holdout evaluation path, while Swift always
evaluates holdouts for every feature.

## Problem

Python had two different code paths:
1. WITH holdouts: get_variation_for_feature → get_decision_for_flag
2. WITHOUT holdouts: get_variation_for_feature → get_variations_for_feature_list

This created inconsistency and potential bugs.

## Solution

Now Python ALWAYS goes through get_decision_for_flag:
- get_variation_for_feature → get_decision_for_flag (always)
- get_decision_for_flag checks holdouts → experiments → rollouts

## Changes

optimizely/decision_service.py:
- Removed conditional branching in get_variation_for_feature()
- Always call get_decision_for_flag() regardless of holdout existence
- Updated docstring to reflect Swift SDK alignment

## Alignment with Swift SDK

Swift flow:
  getVariationForFeature → getVariationForFeatureList → getDecisionForFlag

Python flow (now):
  get_variation_for_feature → get_decision_for_flag

Both now:
- Always evaluate holdouts first (even if empty list)
- Use single unified code path
- No conditional branching

## Testing

✓ Features without holdouts work correctly
✓ Features with holdouts work correctly
✓ Features with experiments work correctly
✓ Unified code path verified

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: muzahidul-opti

optimizely/decision_service.py

@@ -659,6 +659,9 @@ def get_variation_for_feature(
     ) -> DecisionResult:
         """ Returns the experiment/variation the user is bucketed in for the given feature.
 
+        Aligned with Swift SDK which always evaluates holdouts for every feature.
+        Swift: getVariationForFeature → getVariationForFeatureList → getDecisionForFlag (always)
+
         Args:
           project_config: Instance of ProjectConfig.
           feature: Feature for which we are determining if it is enabled or not for the given user.
@@ -671,13 +674,11 @@ def get_variation_for_feature(
             - 'error': Boolean indicating if an error occurred during the decision process.
             - 'reasons': List of log messages representing decision making for the feature.
         """
-        holdouts = project_config.get_holdouts_for_flag(feature.key)
-
-        if holdouts:
-            # Has holdouts - use get_decision_for_flag which checks holdouts first
-            return self.get_decision_for_flag(feature, user_context, project_config, options)
-        else:
-            return self.get_variations_for_feature_list(project_config, [feature], user_context, options)[0]
+        # CRITICAL FIX: Always call get_decision_for_flag (matching Swift SDK behavior)
+        # Swift always goes through getDecisionForFlag which checks holdouts first,
+        # then experiments, then rollouts - regardless of whether holdouts exist.
+        # Previously Python had conditional logic that created two different code paths.
+        return self.get_decision_for_flag(feature, user_context, project_config, options)
 
     def get_decision_for_flag(
         self,