feat(antigravity): update default model to Claude Opus 4.6 (#10720)

* feat(antigravity): update default model to Claude Opus 4.6

Claude Opus 4.5 has been replaced by Claude Opus 4.6 on the
Antigravity (Google Cloud Code Assist) platform.

- Update DEFAULT_MODEL in google-antigravity-auth extension
- Update testing docs to reference the new model

* fix: update remaining antigravity opus 4.5 refs in zh-CN docs and tests

Address review comments from Greptile:
- Update zh-CN/testing.md antigravity model references
- Update pi-tools-agent-config.test.ts model IDs

* Antigravity: default OAuth model to Opus 4.6 (#10720) (thanks @calvin-hpnet)

---------

Co-authored-by: Tak Hoffman <781889+Takhoffman@users.noreply.github.com>

extensions/google-antigravity-auth/index.ts

@@ -13,7 +13,7 @@ const REDIRECT_URI = "http://localhost:51121/oauth-callback";
 const AUTH_URL = "https://accounts.google.com/o/oauth2/v2/auth";
 const TOKEN_URL = "https://oauth2.googleapis.com/token";
 const DEFAULT_PROJECT_ID = "rising-fact-p41fc";
-const DEFAULT_MODEL = "google-antigravity/claude-opus-4-5-thinking";
+const DEFAULT_MODEL = "google-antigravity/claude-opus-4-6-thinking";
 
 const SCOPES = [
   "https://www.googleapis.com/auth/cloud-platform",