Use portable hash for identity override virtual segment keys

[emyller]

Identity override virtual segment keys use Java's hashCode() method, producing non-portable values across SDK implementations.

Acceptance criteria

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

Technical context

Current implementation uses non-portable hash:

flagsmith-java-client/src/main/java/com/flagsmith/mappers/EngineMappers.java

Line 444 in b909755

return String.valueOf(keyBuilder.toString().hashCode());

Portable standard (JSON + SHA256):

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

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