Prevent sending messages in chat if the previous response hasn't finished

[hsubox76]

Right now, if the user calls sendMessage or sendMessageStream before the last exchange has completed, we just put it in a queue (promise chain) and send it when the last response completes. This can cause unexpected output, as the user perceives that they sent two messages in a row with no response in between (history is user-user-model), whereas the backend actually responded to the first user message before seeing the second one (user-model-user). We should throw an error (or no-op and warn) to let the user know they are trying to send a message before the last response completed, and may get unexpected results.