Improve Telegram draft preview startup

[tobi]

Summary

• open an empty Telegram draft as soon as a Telegram turn starts so clients show draft/thinking state before first streamed token
• keep the typing indicator active until after the final reply is sent
• rely on Telegram sendMessage to auto-clear the draft instead of sending an explicit empty draft clear at finalization

Testing

• tested locally through the pi Telegram bridge with repeated Telegram prompts