[Interactions v1] HTTP 400 for documented text + inline PNG Content array — model naming, free-tier support, and image constraints?

Hello Google Gemini API team,

I am seeking a contract clarification for the stable Gemini Interactions v1 REST API. I am not asking for account, credential, quota, or billing troubleshooting, and I have intentionally omitted all secrets, project identifiers, request identifiers, raw provider messages, prompt content, and image bytes.