Move the fenced-code-block-aware splitting logic out of the shared
split_message helper (used by Signal, Slack, Discord, Weixin, etc.)
and into a Telegram-specific _split_telegram_markdown function.
The shared split_message remains a plain-text chunker. The Telegram
channel now uses _split_telegram_markdown for its raw Markdown paths
that feed _markdown_to_telegram_html, preventing broken HTML rendering
when splits fall inside fenced code blocks.
Also fixes a regression where content beginning with whitespace before
a fence could emit a whitespace-only chunk.
Addresses review feedback on #4257.
When split_message splits a long message, it now checks whether the
split point falls inside a fenced code block. If so, it either moves
the split to before the opening fence or closes/reopens the fence
across chunks, preventing broken HTML rendering.
Addresses #4250
