feat: route Banana image model aliases by requested size

[miaomiaozii]

Summary

• automatically route Banana requests to the base, 2K, or 4K model alias from the requested size
• preserve the request's original width, height, and size values
• do not add or rewrite image_size
• forward a gateway-compatible aspect_ratio for Banana/Gemini image requests
• limit routing strictly to known model aliases

Verification

• 8 isolated request-routing tests passed
• tests do not depend on local API keys or .env files

This replaces the previously considered SOCKS-related PR scope; no SOCKS changes are included.