fix: skip sandbox endpoints for Gemini CLI models

Gemini CLI models (gemini-3-flash-preview, etc.) only work with the
production endpoint (cloudcode-pa.googleapis.com). Previously, the
fallback loop tried all endpoints including sandbox ones, causing
404/403 errors before reaching production.

This fix skips sandbox endpoints when headerStyle is 'gemini-cli',
eliminating the error cascade reported in issue #233.

Fixes #233