Adopt shared mapper test data from engine-test-data

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

## Acceptance criteria
- [ ] mapper_test_cases pass (see Flagsmith/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:
https://github.com/Flagsmith/flagsmith-java-client/blob/b9097558fa83824068382457d944b721c206d01e/src/main/java/com/flagsmith/mappers/EngineMappers.java#L444

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: https://github.com/Flagsmith/engine-test-data/pull/46

Related issues:
- Flagsmith/flagsmith-python-client#181
- Flagsmith/flagsmith-ruby-client#93
- Flagsmith/flagsmith-nodejs-client#228
- Flagsmith/flagsmith-go-client#196
- Flagsmith/flagsmith-rust-flag-engine#26
- Flagsmith/flagsmith-dotnet-client#180

Start with EngineMappers.java around line 444 and compare its identity override virtual segment key generation with the PHP and .NET references. Review engine-test-data pull request 46 and run the mapper_test_cases. Done means the shared cases pass and key generation matches the JSON plus SHA256 portable standard.

Written by the indexing model from the issue text.