fix: align responses tool conversion with Vertex schema

[bhrum]

Summary

• Treat OpenAI Responses developer messages as Gemini system instructions when converting to Gemini requests.
• Stop forwarding OpenAI Responses call_id values as functionCall.id / functionResponse.id, which Vertex Gemini rejects as unknown fields.
• Add request-conversion tests covering both cases.

Why

When Codex uses the OpenAI Responses API through CLIProxyAPI against Vertex Gemini, tool-call history can fail with errors like:

Unknown name "id" at 'contents[].parts[].function_call': Cannot find field.
Unknown name "id" at 'contents[].parts[].function_response': Cannot find field.

Codex can also send developer role messages, while Gemini request contents only accept Gemini roles such as user and model; system/developer instructions should be represented via systemInstruction.

Test plan

• go test ./internal/translator/gemini/openai/responses ./sdk/api/handlers/openai

[github-actions]

This pull request targeted main.

The base branch has been automatically changed to dev.

[gemini-code-assist]

Code Review

This pull request updates the OpenAI to Gemini response request translator to support the 'developer' role by mapping it to Gemini's system instructions. It also removes the forwarding of function call and response IDs to the Gemini API. Corresponding unit tests have been added to ensure the 'developer' role is correctly handled and that function IDs are no longer included in the output. I have no feedback to provide.