Route model inference through Cloudflare AI Gateway

[matthewp]

Changes

• Routes Workers AI and Anthropic models exclusively through the shared Cloudflare AI Gateway using Factory-scoped Worker secrets.
• Removes direct provider paths and the Workers AI binding, disables payload retention, and translates existing anthropic/… and cloudflare/… config values to gateway routes.

Testing

• Adds provider tests for secret mapping, gateway endpoints, Kimi metadata, and payload-retention headers.
• Updates configuration tests to cover gateway model names, legacy alias translation, and unsupported providers.

Docs

• Updates the README and local secret template for gateway-only model configuration.