In ACCS solution bot service will be involved in chat interaction. Some of them are escalated to the live agent, others are answered by the bot itself. Is there any way to accept and log as an answered contact without any live agent? Additionally would be great if we can add the customer-bot transcript to the customer history as well.
In case chat escalation to live-agent we passed customer-bot conversation to the live agent so this part is OK.

Is there any way to "answer" and close a chat without agent and report it as answered interaction?

Why wouldn't you implement the chat-bot externally and transfer the contact to the agent when needed? If you do, there is no reason the chatbot couldn't post the message to the agent (and thus be included in the transcript) on connection. Likewise I'm unclear why you would want to "fake" an agent interaction; I imagine you wish stats to be included somewhere but what agent would they report against? That said, I believe some solutions do operate as agents and you can do so via CCT or CCS but they require licenses just like real agents.