Requests ending in a role:"system" message get rejected with invalid_message_role

[awcator]

When a request's messages array ends with a role: "system" message (Claude Code sends these), the proxy converts it to an assistant turn and the request fails:

400 invalid_message_role: The last message in a Gemini request must not be from the assistant

Repro:

{"model":"gemini-3.6-flash","messages":[{"role":"user","content":"hi"},{"role":"system","content":"do x"}]}

translateAnthropicMessagesToOpenAI only handles role: "user" and treats everything else as assistant. Suggest folding role:"system" text into the system prompt instead.

[caozhiyuan]

@awcator use https://github.com/caozhiyuan/copilot-api/