Fix OpenAI OAuth image n fanout

[SipengXie2024]

Summary

• stop passing unsupported tools[0].n to the Responses image_generation tool
• fan out non-streaming OAuth image requests with n > 1 into repeated n=1 upstream calls and aggregate the Images API data[]/usage response
• reject streaming OAuth image requests with n > 1 explicitly because streaming fanout cannot preserve a standard stream contract

Tests

• GOCACHE=/tmp/go-build-sub2api GOMODCACHE=/tmp/go-mod-sub2api go test ./internal/service -run 'TestOpenAIGatewayServiceForwardImages_OAuth(FanoutNAndReturnsAllImages|RejectsStreamingN|NonStreamModerationBlockedReturnsClientError)'
• GOCACHE=/tmp/go-build-sub2api GOMODCACHE=/tmp/go-mod-sub2api go test ./internal/service currently fails on unrelated TestRunProxyQualityTarget_CloudflareChallenge

[github-actions]

Thank you for your contribution! Before we can merge this PR, we need you to sign our Contributor License Agreement (CLA).

To sign, please reply with the following comment:

I have read the CLA Document and I hereby sign the CLA

You only need to sign once — it will be valid for all your future contributions to this project.

---

I have read the CLA Document and I hereby sign the CLA

---

_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}