add image generation content model

[rhan-oai]

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• add deferred image content reads #21238
• add thread turn item paging #21237
• -> add image generation content model #21236

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Put image content on imageGeneration in schemas

The root JSON schema fixture attaches the new content property to ImageViewThreadItem, while ImageGenerationThreadItem below still lacks it. Regenerating the schemas puts this block under imageGeneration, and cargo test -p codex-app-server-protocol fails with a fixture mismatch. Published schema clients would validate the wrong item shape.

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