[Feature]: Support Google Antigravity SDK as provider

## Summary

Support Google Antigravity SDK as provider

## Component(s) Affected

- [x] Core / Backend (app, agents, config, providers, utils, local_models)
- [ ] Console (frontend web UI)
- [ ] Channels (DingTalk, Feishu, QQ, Discord, iMessage, etc.)
- [ ] Skills
- [ ] CLI
- [ ] Documentation (website)
- [ ] Tests
- [ ] CI/CD
- [ ] Scripts / Deploy

## Problem / Motivation

Taking advantages of the new [antigravity-sdk](https://github.com/google-antigravity/antigravity-sdk-python)

## Proposed Solution

## Alternatives Considered

## Additional Context

## Willing to Contribute

- [ ] I am willing to open a PR for this feature (after discussion).