Fix four CQRS pipeline bugs in Vulthil.SharedKernel.Application (T01)

[Vulthil]

Summary

• Fixes T01 (docs/design/tasks/T01-application-validation-and-registration.md): four bugs in the
  Application-layer CQRS pipeline, two independently confirmed, two re-verified from reports.
• ValidationPipelineBehavior never actually returned a failed Result on validation failure — it
  always fell through to throw ValidationException, contradicting its own XML doc.
• HandlerRegistrar only registered the first IHandler<,> a class implemented; a class implementing
  two handler interfaces silently dropped the second.
• Handlers created via ActivatorUtilities were never disposed; InnerHandlerAdapter now forwards
  disposal (registering the concrete type in DI directly was rejected — it would make the handler
  resolvable outside the pipeline, breaking a documented invariant and an existing test).
• RequestLoggingPipelineBehavior serialized errors using the static Error type, dropping
  ValidationError's inner Errors list from the log scope.

Test plan

• dotnet build src/Vulthil.SharedKernel.Application — net9.0 & net10.0, 0 warnings
• Vulthil.SharedKernel.Application.Tests — 26/26 pass on net9.0 and net10.0
• New tests confirmed to fail against the old (unfixed) source, pass against the fix
• Vulthil.SharedKernel.Tests (11/11) and Vulthil.Results.Tests (192/192) pass
• PublicAPI.Shipped/Unshipped.txt unchanged across the repo

[claude]

Code review

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

[Vulthil]

Successfully created backport PR for v1.0:

• [Backport v1.0] Fix four CQRS pipeline bugs in Vulthil.SharedKernel.Application (T01) #282