OpenAI: preserve Chat Completions response_format when forwarding to Responses

[zhuhaow]

What changed

• preserve Chat Completions response_format when requests are forwarded through the Responses API
• keep structured output schemas intact for both OpenAI API key and OpenAI OAuth accounts
• add regression tests for request conversion and forwarding

Why

• Chat Completions requests were dropping response_format during Chat -> Responses translation
• that caused structured output schemas to be lost before the upstream request was sent

Testing

• GOCACHE=/tmp/sub2api-go-build GOMODCACHE=/tmp/sub2api-go-mod go test ./internal/pkg/apicompat ./internal/service -run 'TestChatCompletionsToResponses_ResponseFormat|TestOpenAIGatewayService_ForwardAsChatCompletions_(APIKey|OAuth)PreservesStructuredOutput'