[AI-FSSDK] [FSSDK-12337] Add Feature Rollout support

Add Feature Rollout support to the Ruby SDK. During project config
parsing, inject the "everyone else" variation from the flag's rollout
into any experiment with type "feature_rollout", enabling correct
evaluation without changes to decision logic.

- Added config parsing logic to inject the everyone else rollout
  variation into feature_rollout experiments
- Added traffic allocation entry (endOfRange=10000) for injected variation
- Added get_everyone_else_variation helper to extract the last rollout
  rule's first variation
- Added 6 unit tests covering injection, variation maps, edge cases,
  and backward compatibility

Author: FarhanAnjum-opti

lib/optimizely/config/datafile_project_config.rb

@@ -205,6 +205,28 @@ def initialize(datafile, logger, error_handler)
         feature_flag['experimentIds'].each do |experiment_id|
           @experiment_feature_map[experiment_id] = [feature_flag['id']]
         end
+
+        # Feature Rollout support: inject the "everyone else" variation
+        # into any experiment with type == "feature_rollout"
+        everyone_else_variation = get_everyone_else_variation(feature_flag)
+        next if everyone_else_variation.nil?
+
+        feature_flag['experimentIds'].each do |exp_id|
+          experiment = @experiment_id_map[exp_id]
+          next unless experiment && experiment['type'] == 'feature_rollout'
+
+          experiment['variations'].push(everyone_else_variation)
+          experiment['trafficAllocation'].push(
+            'entityId' => everyone_else_variation['id'],
+            'endOfRange' => 10_000
+          )
+          @variation_key_map[experiment['key']][everyone_else_variation['key']] = everyone_else_variation
+          @variation_id_map[experiment['key']][everyone_else_variation['id']] = everyone_else_variation
+          @variation_id_map_by_experiment_id[exp_id][everyone_else_variation['id']] = everyone_else_variation
+          @variation_key_map_by_experiment_id[exp_id][everyone_else_variation['key']] = everyone_else_variation
+          variation_variables = everyone_else_variation['variables']
+          @variation_id_to_variable_usage_map[everyone_else_variation['id']] = generate_key_map(variation_variables, 'id') if variation_variables
+        end
       end
 
       # Adding Holdout variations in variation id and key maps
@@ -690,6 +712,38 @@ def get_holdout(holdout_id)
 
     private
 
+    def get_everyone_else_variation(feature_flag)
+      # Get the "everyone else" variation for a feature flag.
+      #
+      # The "everyone else" rule is the last experiment in the flag's rollout,
+      # and its first variation is the "everyone else" variation.
+      #
+      # feature_flag - Feature flag hash
+      #
+      # Returns the "everyone else" variation hash, or nil if not available.
+
+      rollout_id = feature_flag['rolloutId']
+      return nil if rollout_id.nil? || rollout_id.empty?
+
+      rollout = @rollout_id_map[rollout_id]
+      return nil if rollout.nil?
+
+      experiments = rollout['experiments']
+      return nil if experiments.nil? || experiments.empty?
+
+      everyone_else_rule = experiments.last
+      variations = everyone_else_rule['variations']
+      return nil if variations.nil? || variations.empty?
+
+      variation = variations.first
+      {
+        'id' => variation['id'],
+        'key' => variation['key'],
+        'featureEnabled' => variation['featureEnabled'] == true,
+        'variables' => variation.fetch('variables', [])
+      }
+    end
+
     def generate_feature_variation_map(feature_flags)
       flag_variation_map = {}
       feature_flags.each do |flag|