Coerces arbitrary string input into HTML suitable for TipTap (or similar) content.
If the trimmed string opens with a known-safe HTML tag, it is sanitized and returned.
Otherwise the string is treated as plain text: split on blank lines into paragraphs,
HTML-escaped, with single newlines inside a block rendered as <br />.
Coerces arbitrary string input into HTML suitable for TipTap (or similar)
content.<br />.