Update StrEnum to v2.1.0. Add integration tests for string enums in composite primary keys.

[dmytro-khmara]

Summary

• Updates StrEnum to v2.1.0, which adds IComparable<TEnum>, IComparable, and IEquatable<TEnum> to StringEnum<TEnum>, letting a string enum be used as an EF Core key or as part of a composite key.
• Adds a StrEnum.EntityFrameworkCore.IntegrationTests project, following the *.IntegrationTests convention already used across the ecosystem. It runs against SQLite in memory, so it needs no Docker and runs as part of the existing Docker test target.
• Adds CompositePrimaryKeyTests, covering a string enum as part of a composite primary key: the model shape, a full round-trip through insert, FindAsync on the composite key, update and query, and a raw-SQL check that the key column holds the member's value.

Test plan

• dotnet test — 11 unit and 3 integration tests pass
• docker build --target test . — the CI path passes with the new project wired into the solution and the Dockerfile restore layer