Let image generation extension hosts control output persistence

[won-openai]

Why

Some extension hosts need generated images returned without writing them to the local filesystem or giving the model a local path.

What changed

tl;dr: we now conduct all extension operations in the image gen extension

• Let hosts provide an optional image save root when installing the extension.
• Save images and return path hints only when a save root is configured.
• Return image data without saving or adding a path hint when no save root is configured.
• Preserve the extension-provided saved_path instead of persisting extension images again in core.
• Leave built-in image generation unchanged.

Validation

• just test -p codex-image-generation-extension
• just test -p codex-app-server standalone_image_generation_returns_saved_path_hint_to_model
• just test -p codex-core extension_tool_uses_granted_turn_permissions_without_local_persistence
• just test -p codex-core tools::handlers::extension_tools::tests
• tested on CODEX CLI on both save_root: CODEX_HOME and None
• tested on CODEX APP on both as well

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: fbe68a32ea

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".