Adopt shared mapper test data from engine-test-data

[emyller]

Mapper tests use inline expected context data. When engine-test-data releases shared mapper test cases, this client should adopt them for cross-SDK consistency.

Acceptance criteria

• Mapper test uses expected context from engine-test-data
• mapper_test_cases pass (see feat(Mappers): Add mapper test cases engine-test-data#46)

Technical context

Test data: Flagsmith/engine-test-data#46