Let models opt into original image detail (#14175)

## Summary

This PR narrows original image detail handling to a single opt-in
feature:

- `image_detail_original` lets the model request `detail: "original"` on
supported models
- Omitting `detail` preserves the default resized behavior

The model only sees `detail: "original"` guidance when the active model
supports it:

- JS REPL instructions include the guidance and examples only on
supported models
- `view_image` only exposes a `detail` parameter when the feature and
model can use it

The image detail API is intentionally narrow and consistent across both
paths:

- `view_image.detail` supports only `"original"`; otherwise omit the
field
- `codex.emitImage(..., detail)` supports only `"original"`; otherwise
omit the field
- Unsupported explicit values fail clearly at the API boundary instead
of being silently reinterpreted
- Unsupported explicit `detail: "original"` requests fall back to normal
behavior when the feature is disabled or the model does not support
original detail

codex-rs/core/src/features.rs

@@ -130,7 +130,7 @@ pub enum Feature {
     MemoryTool,
     /// Append additional AGENTS.md guidance to user instructions.
     ChildAgentsMd,
-    /// Allow `detail: "original"` image outputs on supported models.
+    /// Allow the model to request `detail: "original"` image outputs on supported models.
     ImageDetailOriginal,
     /// Enforce UTF8 output in Powershell.
     PowershellUtf8,
@@ -1002,6 +1002,15 @@ mod tests {
         assert_eq!(Feature::ImageGeneration.default_enabled(), false);
     }
 
+    #[test]
+    fn image_detail_original_feature_is_under_development() {
+        assert_eq!(
+            Feature::ImageDetailOriginal.stage(),
+            Stage::UnderDevelopment
+        );
+        assert_eq!(Feature::ImageDetailOriginal.default_enabled(), false);
+    }
+
     #[test]
     fn collab_is_legacy_alias_for_multi_agent() {
         assert_eq!(feature_for_key("multi_agent"), Some(Feature::Collab));