Update antigravity User-Agent from 1.11.5 to 1.15.8

[Daviey]

Google started rejecting requests with the old antigravity client version (1.11.5), returning:

"This version of Antigravity is no longer supported. Please update to receive the latest features!"

The User-Agent in packages/ai/src/providers/google-gemini-cli.ts (line 76) needs bumping to antigravity/1.15.8. Hit this running moltbot with the google-antigravity provider — cleared Node compile cache and confirmed the fix works after patching locally.

Also worth updating the example in packages/coding-agent/examples/extensions/antigravity-image-gen.ts.

Related: #1068, badrisnarayanan/antigravity-claude-proxy#213

[badlogic]

thanks for reporting, fixed