When a Telegram Forum topic is created by the bot, Telegram generates a
system message with from.id=botId and empty text. Every subsequent user
message in that topic has reply_to_message pointing to this system
message, causing the implicitMention check to fire and bypassing
requireMention for every single message.
Add a guard that recognises system messages (is_bot=true with no text)
and excludes them from implicit mention detection, so that only genuine
replies to bot messages trigger the bypass.
Closes#32256
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
