OAuth wrapper for managed providers (v2.1.0)

[typelicious]

Goal

Implement OAuth-based authentication wrapper for "managed providers" (Gemini via Google OAuth, Antigravity, etc.) as part of v2.1.0.

Requirements

• Interactive login flows with device-code or web-auth patterns
• Token refresh and session management
• Wrapper that presents OAuth-secured endpoints as regular API-key providers
• Support for Gemini (Google OAuth), Antigravity, Qwen, OpenAI Codex, Claude Code, and other OAuth-first gateways
• Clear separation from automated routing core (optional feature)

Context

From v2.1.0 planning in FAIGATE-ROADMAP.md:

• Managed provider OAuth wrapper is a core theme
• Should be optional; API-key providers remain the default
• Interactive login flows must be clearly separated from automated routing

Tasks

• Design OAuth wrapper architecture
• Implement Google OAuth flow for Gemini
• Implement Qwen OAuth (free tier device-code flow)
• Support OpenAI Codex (OAuth via ChatGPT)
• Support Claude Code (if available via OAuth)
• Implement generic OAuth provider interface
• Add token storage and refresh logic
• Integrate with provider registry and config system
• Documentation and setup guides
• Testing with real OAuth providers

Notes

• This enables access to providers that require interactive login
• Keeps gateway-first principle while expanding provider coverage
• Should not break existing API-key based providers
• Qwen, OpenAI Codex, and Claude Code are important additions for complete LLM AI Router parity