[AI-FSSDK] [FSSDK-12760] add localHoldouts to datafile for backward compatibility

[jaeopt]

Summary

Adds support for the new top-level localHoldouts datafile section so this SDK can be served the same backward-compatible datafile shape that Python (#517) and Ruby (#400) already consume.

Section membership — not includedRules — is now the sole signal for holdout scope:

• holdouts → ALL entries are global. Any stray includedRules on these entries is stripped at parse time and ignored.
• localHoldouts → ALL entries are local. Entries without a non-empty includedRules list are invalid and skipped with an error log (no fallback to global application).

Old datafiles that emit only holdouts continue to parse unchanged.

Changes

• Sources/Data Model/Project.swift — decode the new top-level localHoldouts array; default to [] when absent. Include it in CodingKeys and Equatable.
• Sources/Data Model/HoldoutConfig.swift — section-aware mapping. New init(globalHoldouts:localHoldouts:). Global-section entries get includedRules stripped; local-section entries without includedRules are logged and excluded. Legacy init(allholdouts:) and allHoldouts setter are preserved (they partition by includedRules == nil and then apply the same section semantics) so existing tests/fixtures keep working.
• Sources/Data Model/ProjectConfig.swift — construct HoldoutConfig from both project.holdouts and project.localHoldouts sections.
• Sources/Data Model/Holdout.swift — docstring on isGlobal clarifies that section membership is the source of truth; HoldoutConfig strips includedRules from holdouts-section entries so the property stays consistent.

Tests

• Tests/OptimizelyTests-DataModel/HoldoutConfigTests.swift — new tests for the section-aware semantics: global-section includedRules are stripped/ignored, local section without includedRules is excluded, mixed valid/invalid local entries, both-sections partition enforced, backward compat with no localHoldouts section.
• Tests/OptimizelyTests-DataModel/ProjectTests.swift — new tests for localHoldouts decoding (missing key defaults to []; present key decodes alongside holdouts).
• Tests/OptimizelyTests-DataModel/ProjectConfigTests.swift — updated existing testGenerateProjectConfigMapWithHoldouts to put local holdouts in the new localHoldouts section (since includedRules on holdouts-section entries is now stripped).

Reference PRs

• Python: [AI-FSSDK] [FSSDK-12760] Add localHoldouts datafile section for backward compat python-sdk#517
• Ruby: [AI-FSSDK] [FSSDK-12760] Add localHoldouts datafile section ruby-sdk#400

Jira

FSSDK-12760