fix: restrict YouTube resolver URLs

[vycdev2]

Summary

• reject external and deceptive hosts before resolving user-supplied YouTube channel references
• normalize supported YouTube references to canonical URLs and discard unrelated query/fragment values
• add regression coverage for external URLs, schemeless references, redirect paths, channel IDs, handles, subdomains, and youtu.be links

Verification

• dotnet build — passed with 0 errors (existing NU1903 advisory warning remains)
• dotnet test — passed, 204 tests
• git diff --check — passed

Risk

• Low: the change is isolated to YouTube reference normalization; documented channel ID, channel URL, handle, custom URL, and user URL forms remain covered, along with existing youtu.be handling.

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