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Claude Opus 4.5 has been replaced by Claude Opus 4.6 on the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>

src/agents/pi-tools-agent-config.test.ts

@@ -148,7 +148,7 @@ describe("Agent-specific tool filtering", () => {
       workspaceDir: "/tmp/test-provider",
       agentDir: "/tmp/agent-provider",
       modelProvider: "google-antigravity",
-      modelId: "claude-opus-4-5-thinking",
+      modelId: "claude-opus-4-6-thinking",
     });
 
     const toolNames = tools.map((t) => t.name);
@@ -176,7 +176,7 @@ describe("Agent-specific tool filtering", () => {
       workspaceDir: "/tmp/test-provider-profile",
       agentDir: "/tmp/agent-provider-profile",
       modelProvider: "google-antigravity",
-      modelId: "claude-opus-4-5-thinking",
+      modelId: "claude-opus-4-6-thinking",
     });
 
     const toolNames = tools.map((t) => t.name);