[AI-FSSDK] [FSSDK-12792] Add localHoldouts datafile section for backward-compatible local holdout support

[junaed-optimizely]

Summary

• Add new top-level localHoldouts datafile section to separate local (rule-scoped) holdouts from global holdouts
• Strip includedRules from global holdouts at parse time — section membership is the sole signal for scope
• Validate local holdouts: entries without includedRules are logged as errors and excluded
• Enforce forced decision priority over local holdouts (mandatory cross-SDK ordering)

Changes

• DatafileProjectConfig.cs: Parse localHoldouts from datafile; strip IncludedRules from global holdouts; validate local holdouts
• ProjectConfig.cs: Add LocalHoldouts property to IProjectConfig interface
• Holdout.cs: Change IsGlobal to settable property with internal setter; update docstrings
• HoldoutConfig.cs: Use holdout.IsGlobal property instead of computing from IncludedRules
• DecisionService.cs: Update comment for section-based scope
• Tests: Add parsing tests, validation tests, backward compatibility tests, and mandatory forced-decision-beats-local-holdout test

Jira Ticket

FSSDK-12792