Support OAuth OpenAI login for subscription usage in AI Gateway

### Problem
moltworker currently does not allow OAuth-based OpenAI login to use an existing OpenAI subscription, even though the OpenClaw repository supports this flow.

### Why this matters
Without OAuth login support, usage via Cloudflare is effectively much more expensive for users who already pay for OpenAI subscriptions.

### Request
Please add support for OAuth OpenAI login (subscription-backed usage) in the AI Gateway flow, similar to OpenClaw support.

### User impact
This would reduce duplicate costs and make Cloudflare-hosted usage more practical for existing OpenAI subscribers.