chore: Clean up e2e tests and comprehensively use cassettes#2001

Luca Forstner (lforst) wants to merge 3 commits into

mainbraintrustdata/braintrust-sdk-javascript:mainfrom

lforst/cassette-server-e2ebraintrustdata/braintrust-sdk-javascript:lforst/cassette-server-e2eCopy head branch name to clipboard