fix fix-telegram-group-replies
The bot was silently ignoring group messages because:
- Mention detection only checked plain-text "@botname" but Telegram often sends mentions as entities (MENTION / TEXT_MENTION) when users autocomplete the bot name — these were missed.
- No logging at all — impossible to diagnose why the bot was silent.
Changes:
- Extract _is_bot_mentioned() helper that checks both plain text and Telegram message entities (MENTION and TEXT_MENTION types), with case-insensitive username comparison.
- Add debug/info logging throughout _on_message: chat ID, chat type, allowed-chat filtering, mention/reply detection results.
- Log allowed-chats configuration at startup.
- Import MessageEntityType from telegram.constants.
- Add comprehensive tests for _is_bot_mentioned.