[codex] Detach image upstream context

[cyeinfpro]

Summary

• Detach OpenAI Images API key upstream requests from the client request context.
• Add a regression test covering non-stream image forwarding after the client context is canceled.

Why

For non-stream OpenAI Images requests, the upstream request used the client request context. If the client or an intermediate proxy disconnected before the upstream image generation completed, the upstream request was canceled with context canceled. That prevented the gateway from receiving a successful image response and skipped the existing RecordUsage billing path, even though the upstream provider could already have accepted the request.

Streaming image requests and the Responses-based image path already use detached upstream contexts. This change makes the API key non-stream Images path consistent with those flows.

Validation

go test ./internal/service -run 'TestOpenAIGatewayServiceForwardImages_APIKey(GenerationUsesConfiguredV1BaseURL|NonStreamDetachesUpstreamContext|StreamJSONResponseBillsImage|StreamingDrainsAfterClientDisconnect)$'

Also ran git diff --check.

[github-actions]

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