Handle running out of context length gracefully

[sutyum]

• [x] Automatically switch from gpt-4 to gpt-3.5-16k and from gpt-3.5-16k to gpt-3.5-32k
• [x] If the context size is still exceeded, Show an error UI stating the chat has reached its maximum length

[PrinceBaghel258025]

• the last message will be appended to the chat data
• in this was it will be shown at the bottom just above the input box
• start a new chat will appear there as well
• model switching button will be disabled
• input box, chat-with-pdf and send button will also be disabled