Finish connecting a Google Antigravity account started with add_antigravity_start.
`code` is the value of `?code=...` on the antigravity.google/oauth-callback page the
user landed on; `state` is what add_antigravity_start returned. Returns
{status, account_id, account_email, requires_project}. By default this also creates
the GCP project in one step (`complete=True`) and adds `project_id`. Pass
`complete=False` to defer and call `complete_antigravity_account(account_id)` later.
If auto-onboarding fails the result keeps `requires_project: true` and carries
`onboard_error`; retry with `complete_antigravity_account(account_id)`. Errors:
"unknown state" (start again) or "google_api_error" (the code was rejected or
already used; start again).
ConnectorNo auth