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)
• Remove inline expected context JSON from MappersTest.cs

Technical context

Current implementation uses inline JSON:
https://github.com/Flagsmith/flagsmith-dotnet-client/pull/177/files#diff-f851c822fa0e162b75312b6b9ab5dc31d3631179a33360ec77bbb83f4ae7c50eR12-R140

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

Related issues:

• Adopt shared mapper test data from engine-test-data flagsmith-python-client#181
• 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