feat(codex): support image references for Responses imagegen and add playground UI

[0x0079]

Motivation

• Add support for sending reference images to the Responses-based image generation flow so Codex-backed imagegen can accept image references in addition to text prompts.
• Improve the Playground UX so users can provide reference image URLs when experimenting with image generation.
• Keep the change additive and backward-compatible so existing imagegen calls without references continue to work.

Description

• Update buildImageGenerationResponsesRequest to append input_image content items (with image_url) after the prompt when reference URLs are present in the ImageGenerateParams payload.
• Add helpers extractImageReferencesFromImageGenerateRequest and normalizeStringSlice to parse input_image_refs, image_urls, or reference_images arrays from the image generation request body and normalize them safely.
• Update the Playground at frontend/src/pages/scenario/PlaygroundPage.tsx to add imageRefs state, a multiline text input for one-URL-per-line references, and forward references via extra_body.input_image_refs in client.images.generate(...).
• Formatting and minor housekeeping applied (gofmt).

Testing

• Ran gofmt -w internal/client/codex_client.go successfully.
• Ran frontend lint with npm --prefix frontend run -s lint -- frontend/src/pages/scenario/PlaygroundPage.tsx, which completed successfully but reported unrelated pre-existing warnings.
• Attempted go test ./internal/client -run TestCodex -count=1, which failed in this environment due to a missing local replacement module (libs/anthropic-sdk-go) unrelated to the changes, so unit tests could not be fully executed here.

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Codex Task