fix: propagate YouTube feed cancellation

[vycdev2]

Summary

• propagate caller-requested cancellation from YouTube RSS feed fetches instead of treating it as a feed failure
• add a regression test using an already-cancelled token

Verification

• dotnet test --filter FullyQualifiedName~YoutubeFeedServiceTests --no-restore — passed: 1 test
• dotnet build — passed with the existing NU1903 package vulnerability warning
• dotnet test --no-build — passed: 189 tests
• git diff --check — passed

Risk

• Low: the behavior changes only when the supplied cancellation token requested cancellation; other fetch and parsing errors remain logged and converted to an empty result.

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