OptionalmaxMax UTF-16 code units per part (capabilities.maxTextLength).
OptionalmaxMax UTF-8 bytes per part (capabilities.maxTextBytes, e.g. Instagram).
OptionalformatMarkup to keep intact (ctx.textFormat). Unset = Markdown-like (*, _, ~,
backticks, links); 'plain' = no markup.
Limits and markup of splitMessage /
findSplitPoint.