Complete e2e tests for `ask-the-documents`

[cprecioso]

Originally posted by @infomiho in #3250 (comment)

We should test out the full flow of the ask-the-documents app. Probably by mocking the Google and OpenAI communication somehow (MSW, MitM?)

[Martinsos]

Ok, it is important to have good test.
Let's brainstorm more on concrete solutions, try to visalize them a bit, then we will decide how to proceed.