test: add first-party unit tests for Vulthil.xUnit and Vulthil.Extensions.Testing

[Vulthil]

Summary

• Add tests/Vulthil.Extensions.Testing.Tests covering Polling.WaitAsync (both the generic and
  non-generic cores: immediate success, success-on-Nth-attempt, timeout error aggregation,
  cancellation-vs-timeout distinction) and HttpResponseMessageExtensions.GetResponseAsync<T>.
• Add tests/Vulthil.xUnit.Tests covering BaseUnitTestCase lifecycle (lazy Target timing,
  Use/UseReal/UseRealFor, Initialize/Dispose hook ordering, disposal ordering), HttpMock rule
  matching (URL, wildcard, headers, the predicate On(...) overload, unmatched requests, reset),
  SanitizeDatabaseName edge cases, and ContainerHost bookkeeping via fakes (no Docker).
• Relocate the existing BaseUnitTestCaseDisposalTests from Vulthil.SharedKernel.Tests into the
  new project, since its subject is a Vulthil.xUnit type.
• Wire both new projects into the solution under a new /tests/Testing/ folder, mirroring the
  existing /src/Testing/ grouping.

Verification

• Full solution build: clean, 0 warnings/0 errors, both net9.0 and net10.0.
• Vulthil.Extensions.Testing.Tests: 16/16 passing on both TFMs.
• Vulthil.xUnit.Tests: 45/45 passing on both TFMs.
• Vulthil.SharedKernel.Tests (post-relocation): 11/11 passing on both TFMs.
• No public API changes: every PublicAPI.*.txt and CompatibilitySuppressions.xml is unchanged.

Backport to v1.0: no — test-only additions ship with the next release.

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.