Avoid rendering generated images twice (#34378)

## What changed

- Tell the model that generated images are already displayed to the user and should not be repeated as Markdown images or file links.
- Keep generated-image artifact paths and output hints owned by the image generation extension.
- Centralize the availability check for the image generation tool across feature, provider, namespace-tool, model modality, and authentication requirements.

## Testing

- Verify artifact paths sanitize session and call identifiers.
- Verify the model-facing output hint reports that the generated image is already displayed.

GitOrigin-RevId: 6c865b6a96d6a71bb2c33246d1518a00e7c8b840

codex-rs/ext/image-generation/src/lib.rs

@@ -1,3 +1,4 @@
+mod artifact;
 mod backend;
 mod extension;
 mod tool;