sloproxy

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

- 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.


+129 -13
2 changed files