feat: integrate gemini through the same login flow as gemini cli

[mgpai22]

Summary

• Adds Gemini CLI OAuth login flow integration
• Implements PKCE-based OAuth for secure browser authentication
• Handles token exchange, refresh, and caching automatically
• Supports both interactive and headless authentication modes

Fixes #8035

Test plan

• [ ] Test OAuth flow opens browser and completes authentication
• [ ] Verify token refresh works after expiry
• [ ] Confirm API requests succeed with obtained credentials
• [ ] Test headless mode fallback