[AI-FSSDK] (DO NOT REVIEW) [FSSDK-12265] Add experiments field to Holdout data model

[muzahidul-opti]

Summary

• Added Experiments []string field to Holdout struct in both datafile entities and runtime entities
• Added IsLocal() bool method to Holdout that returns true if the experiments slice is not empty
• Added experimentHoldoutsMap to DatafileProjectConfig for mapping experiment IDs to holdouts
• Updated holdout mapper to build experiment-to-holdout mappings
• Added GetHoldoutsForExperiment(experimentID string) method to retrieve holdouts for a specific experiment
• Added GetHoldoutList() method to retrieve all holdouts
• Implemented backward compatibility with omitempty JSON tag for the experiments field
• Added comprehensive unit tests covering:
  • Experiments field serialization and deserialization
  • IsLocal() method functionality
  • Experiment-to-holdout mapping
  • Multiple experiments per holdout
  • Empty experiments arrays
  • Backward compatibility with datafiles without experiments field

Changes

• pkg/config/datafileprojectconfig/entities/entities.go: Added Experiments field to datafile Holdout struct
• pkg/entities/experiment.go: Added Experiments field and IsLocal() method to runtime Holdout struct
• pkg/config/datafileprojectconfig/mappers/holdout.go: Updated mapper to build experiment-to-holdout mappings
• pkg/config/datafileprojectconfig/config.go: Added experimentHoldoutsMap, GetHoldoutsForExperiment(), and GetHoldoutList() methods
• pkg/config/datafileprojectconfig/config_test.go: Added comprehensive unit tests
• pkg/config/datafileprojectconfig/mappers/holdout_test.go: Added tests for experiment mapping

Test Results

All tests pass successfully:

• TestHoldout_ExperimentsField ✓
• TestHoldout_IsLocal ✓
• TestGetHoldoutsForExperiment ✓
• TestExperimentHoldoutsMapping_MultipleExperiments ✓
• TestExperimentHoldoutsMapping_EmptyExperiments ✓
• TestHoldout_BackwardCompatibility ✓
• TestHoldout_JSONSerialization ✓
• TestMapHoldoutsWithExperiments ✓
• TestMapHoldoutsMultipleExperiments ✓

Jira ticket: FSSDK-12265