fix: strip x-goog-user-project header to enable opus 4.6 on Daily endpoint

- Strip x-goog-user-project header ONLY for antigravity headerStyle (Daily endpoint)
- Keep the header for gemini-cli style (Prod endpoint) where it may be needed for billing/quota
- Add test coverage for both headerStyle behaviors

Root cause:
The x-goog-user-project header (added by OpenCode/AI SDK) causes 403 Forbidden
errors on the Daily sandbox endpoint. This triggers fallback to Prod endpoint,
but claude-opus-4-6-thinking is only available on Daily, resulting in 404 errors.

By stripping this header only for antigravity requests, Daily endpoint works
while preserving potential billing/quota functionality for gemini-cli requests.

Fixes #410