txt/html/md export joined and string-munged message.content directly, so a
multimodal turn (content is a list of blocks) crashed export with a TypeError
on join (txt) / AttributeError on .replace (html), and None content (tool-only
assistant turns) rendered as the literal 'None'. Add a _content_to_text helper
that flattens string/list/None to plain text and apply it at the three export
sites. JSON export is unchanged (it serializes structured content correctly).
Plain-string content is returned unchanged, so existing exports are identical.
Co-authored-by: ghreprimand <203024559+ghreprimand@users.noreply.github.com>
