Add client result unions for happy-path and problem responses

[tiksn]

Summary

• Add ClientResult<'T> and ClientUnitResult beside ProblemDetailsModel for typed happy-path and problem responses
• Update IHttpRequestSender, IHttpTransport, and all bridge clients to return result unions instead of raw payloads or unit tasks
• Classify 2xx responses as success and deserialize non-2xx bodies into ProblemDetailsModel
• Add tests for result helpers, transport behavior, and status-code handling

Testing

• dotnet test passed
• ./pack.ps1 passed and produced NuGet and NPM package artifacts