Improve chat history handling

[drivian]

Chat history is silently truncated when going over max_token_limit, as we are using the LangChain class ConversationTokenBufferMemory for the chat_history object. The default max_token_limit in agent.yml is currently set to 4k, which is easily too little with current LLMs. Propose to:

• Increase the limit to say 20k, and change default llm to gpt-4o
• Add a logger if the limit is exceeded (maybe LangChain support this now?)