Centralize the per-platform mb_substr truncation that every publisher was
repeating into MediaItem::altTextFor(Platform), delete each publisher's
private altFor() helper, and clarify the Platform::altTextMaxLength()
docblock so it doesn't imply Instagram's documented 1000-char cap is a
guess. Add the assertions review flagged as missing: LinkedInPage/
InstagramFacebook alt-text caps, non-string and literal-"0" alt_text
normalization, altTextFor() truncation/unsupported-platform behavior,
Mastodon's no-description-part case, and the public API's accept/reject
path for media.*.meta.alt_text.
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