Standardize and refactor LLM response objects in the code

[ianarawjo]

There are multiple different ways responses are being passed around in the code, and it would be nice to standardize this all to the StandardLLMResponse type in TypeScript. Relatedly, we should reconsider the notion of "grab responses" and "fetch from backend" --this made a lot of sense when it was the Python backend, but now it just seems unnecessarily indirect.