Wrap the TelegramAdapter import in _send_to_platform() with a try/except
ImportError guard, matching the existing Feishu pattern in the same function.
When python-telegram-bot is not installed, the import no longer crashes the
cron scheduler. Instead, MAX_MESSAGE_LENGTH falls back to a hardcoded 4096.
The _send_telegram() function already had its own ImportError guard for the
telegram package; this fixes the remaining bare import of TelegramAdapter
in the platform-routing function.
