Adds a vitest case for saveOnboardingLocalEndpoint that mocks
/api/providers/validate answering with `resolved_base_url` set to the
`/v1` variant of a bare host root typed by the user, and asserts the
subsequent /api/model/set body carries that resolved URL rather than the
URL as typed. This guards the salvaged #65489 behaviour that Save stores
the base that actually served /models, since the runtime POSTs
{base_url}/chat/completions verbatim and a host root that only
"detected" via /v1/models would 404 every chat.
588bc72bc8