- Add `ExchangeAntigravityCode`, `FetchAntigravityUserInfo`, and `CompleteAntigravityOAuth` helpers for programmatic authentication.
- Introduce `BuildAntigravityAuthURL` and `AntigravityDefaultCallbackURI` to support custom and default redirect flows.
- Refactor Antigravity credential record assembly into `BuildAntigravityAuth`.
Closes: #5593