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-ruby-client/blob/287ce6d173cf72564da88b8e9c98d042a1deb941/lib/flagsmith/engine/mappers/identity.rb#L38

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-java-client#197
- Flagsmith/flagsmith-nodejs-client#228
- Flagsmith/flagsmith-go-client#196
- Flagsmith/flagsmith-rust-flag-engine#26
- Flagsmith/flagsmith-dotnet-client#180

Inspect lib/flagsmith/engine/mappers/identity.rb at the linked implementation and review the shared mapper_test_cases from Flagsmith/engine-test-data#46. Compare the current identity override virtual segment key generation with the JSON + SHA256 standard described in the linked PHP and .NET changes. Done means mapper_test_cases pass and the generated key matches the portable standard.

索引モデルが issue の本文から書いたものです。