Adds --chat-mode group|topic to lark-cli im +chat-create so users and AI agents can create 话题群 (topic chats) directly via the CLI. Without this, requests to create a topic chat silently fall back to a normal conversation group. Default remains group; chat_mode is now always emitted in the POST /open-apis/im/v1/chats request body.
Change-Id: I79385e2e8606f84e3f27de240d1b41037bf51261