fix: support youtu.be short URLs in YouTubeConverter.accepts()

[kuishou68]

Closes #1730

Problem

YouTubeConverter.accepts() only matched https://www.youtube.com/watch? URLs. Any youtu.be/<id> short URL or youtube.com/shorts/<id> URL returned False, falling through to a generic HTML converter and producing noisy unconverted output.

Additionally, the convert() method extracted the video ID only via parse_qs(urlparse(...).query)["v"], which also fails for short and Shorts URLs.

Fix

• Introduced a standalone helper _extract_youtube_video_id(url) that handles three URL forms:
  • https://www.youtube.com/watch?v=ID (existing)
  • https://youtu.be/ID (short URL — previously broken)
  • https://www.youtube.com/shorts/ID (Shorts — previously broken)
• Updated accepts() to use this helper — it now returns True for all three forms.
• Updated convert() to use the same helper when extracting the video ID for transcript fetching, replacing the old parse_qs approach that silently returned no ID for short URLs.

Signed-off-by: cocoon 54054995+kuishou68@users.noreply.github.com

[kuishou68]

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