[AI-FSSDK] [FSSDK-12760] Add localHoldouts datafile section

[jaeopt]

Summary

Adds a new top-level localHoldouts section to the datafile and updates DatafileProjectConfig to treat section membership as the sole signal for holdout scope. Entries in holdouts are global; entries in localHoldouts are rule-scoped via includedRules.

Changes

• lib/optimizely/config/datafile_project_config.rb
  • Parse new top-level localHoldouts array; expose local_holdouts attribute.
  • All entries in holdouts are treated as global; any includedRules field on those entries is stripped at parse time so it cannot narrow scope.
  • Local holdouts without a non-empty includedRules array are logged at ERROR and excluded from evaluation (no fallback to global).
  • Build variation maps from both global and local holdouts.
  • Updated holdout_global? docstring to reflect section-based scoping.
• lib/optimizely/helpers/constants.rb
  • Added localHoldouts to the datafile JSON schema (mirrors holdouts shape).
• spec/spec_params.rb
  • Moved local-holdout fixtures (holdout_local_1, holdout_local_2) into the new top-level localHoldouts section. Removed the invalid empty-includedRules fixture (now logged + skipped per spec).
• spec/config/datafile_project_config_spec.rb
  • Updated existing tests to use local_holdouts in tandem with holdouts for counts.
  • Added tests: section-based parsing, ignored includedRules on global entries, invalid local holdouts (missing/empty includedRules), backward compatibility with datafiles that have no localHoldouts section.

Backward Compatibility

Older SDKs ignore the unknown localHoldouts key entirely; only the global holdouts section is parsed. Gen 3 SDKs now read both sections with a hard partition. No deserializer risk, no schema enum changes, no log spam.

Jira Ticket

FSSDK-12760