Adopt shared mapper test data from engine-test-data

[emyller]

Mapper tests use inline expected context data rather than shared test cases from engine-test-data repository.

Acceptance criteria

• mapper_test_cases pass (see feat(Mappers): Add mapper test cases engine-test-data#46)
• Identity override virtual segment key generation matches cross-SDK portable standard (JSON + SHA256)

Technical context

Current implementation uses non-portable hash:

flagsmith-python-client/flagsmith/mappers.py

Line 182 in c0d57ec

segment_key = str(hash(overrides_key))

Portable standard (JSON + SHA256):

• PHP: https://github.com/Flagsmith/flagsmith-php-client/pull/124/files#diff-5872f438d0e149e75ba3660c6b8e7ec8af446b87ea6a4e70f45f7e8e9e7b9c4dR208
• .NET: https://github.com/Flagsmith/flagsmith-dotnet-client/pull/177/files#diff-f851c822fa0e162b75312b6b9ab5dc31d3631179a33360ec77bbb83f4ae7c50eR249

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

Related issues:

• Adopt shared mapper test data from engine-test-data flagsmith-ruby-client#93
• Adopt shared mapper test data from engine-test-data flagsmith-java-client#197
• Adopt shared mapper test data from engine-test-data flagsmith-nodejs-client#228
• Adopt shared mapper test data from engine-test-data flagsmith-go-client#196
• Adopt shared mapper test data from engine-test-data flagsmith-rust-flag-engine#26
• Adopt shared mapper test data from engine-test-data flagsmith-dotnet-client#180