Add YouTube oEmbed support for link previews

[KallynGowdy]

Summary

This PR adds specialized support for YouTube URLs in the link preview system by integrating YouTube's oEmbed endpoint, providing better metadata extraction for video content.

Key Changes

• YouTube URL Detection: Added isYouTubeHostname() function and YOUTUBE_HOSTNAMES set to identify YouTube URLs across multiple domains (youtube.com, youtu.be, m.youtube.com, youtube-nocookie.com, etc.)
• oEmbed Integration: Implemented _getYouTubeOEmbedData() method to fetch preview data from YouTube's oEmbed endpoint with proper schema validation using Zod
• Schema Validation: Created YOUTUBE_OEMBED_SCHEMA using Zod to validate and type YouTube's oEmbed responses
• Data Extraction: Added extractYouTubeOEmbedData() function to map oEmbed fields onto the standard LinkPreviewData format
• Refactored Preview Logic: Extracted generic HTML scraping into _getGenericPreviewData() method and introduced PreviewDataResult type to handle both success and failure cases uniformly
• Comprehensive Tests: Added extensive test coverage for YouTube URL detection, oEmbed endpoint integration, error handling, and caching behavior

Implementation Details

• YouTube URLs are detected by hostname and routed to the oEmbed endpoint instead of generic HTML scraping
• The oEmbed response is validated against a schema before use, returning unacceptable_url errors for invalid responses
• Preview data is cached with the same TTL logic as generic previews, respecting cache-control headers
• The implementation maintains backward compatibility with existing generic link preview functionality
• All YouTube-specific code is properly exported for testing and reusability

https://claude.ai/code/session_01PCVUPdxNMWehtCNeGCmLXL

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