Thread replies (messages with root_id != message_id) in group chats
now get their own session key: feishu:{chat_id}:{root_id}. This
means each Feishu thread has an independent conversation context.
Top-level group messages and all private chat messages keep the
default session key (no override), consistent with Telegram and
Slack channel behavior.
Co-authored-by: shenchengtsi <228445050+shenchengtsi@users.noreply.github.com>