feat(management): add request-scoped proxy override for APICall

- Add `proxy_url` to APICall request payload and document it as highest-priority proxy selection.
- Validate request-provided proxy URLs and reject invalid values with `400`.
- Pass request proxy through token resolution/Antigravity token refresh and APICall transport setup.
- Update transport precedence so `proxy_url` overrides credential/global proxies; invalid request proxy falls back to direct.