Fetches a single LinkedIn post or article, returning the title, headline, full description text, author info with follower count, publication date, like count (reactions), comment count, and individual comments. For public feed posts, activityUrn and contentUrn expose LinkedIn's public activity and underlying share or ugcPost URNs when present; either can be null when LinkedIn does not expose it. Also includes related articles from the same author in moreArticles. Uses the caller's OpenAny gateway credentials. Charges and authorization are determined by the gateway.