test: skip unavailable culture regression

[vycdev2]

Summary

• add a culture-aware xUnit theory attribute that explicitly skips tests when their required culture is unavailable
• preserve the Turkish-casing regression test when tr-TR is supported while allowing globalization-invariant test runs to complete

Verification

• dotnet test --filter 'FullyQualifiedName~MiscModuleTests.NormalizeTimeUntilEventName_NormalizesCase' --logger 'console;verbosity=normal' — passed: the unsupported culture was reported as one explicit skip
• dotnet build — passed with the existing SQLitePCLRaw NU1903 advisory warning
• dotnet test --no-build — passed: 297 passed, 1 skipped, 0 failed
• git diff --check upstream/develop...HEAD — passed

Risk

• Low: test-only behavior; the Turkish regression still runs unchanged where that culture is available.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.