Make Keycloak fixture tests reliable

[societ-campbellwray]

Summary

• replace the old Insurance test realm with a canonical keycloak-net-fixture export for Keycloak 26.7.0
• add build/start-keycloak.sh to start/import the fixture realm and optionally run the test suite
• remove guarded assertions across the Keycloak-backed tests and assert against stable fixture data
• add small Keycloak 26.7 compatibility updates for server-info enums and group-by-path normalization
• update README testing instructions for Docker and manual export import workflows

Validation

• dotnet build Keycloak.Net.Core.sln --framework net8.0 --nologo -v:minimal
• ./build/start-keycloak.sh --test --auto-cleanup -- --framework net8.0 --no-build --nologo -v:minimal
  • 124 passed, 5 skipped, 0 failed