Always enable original image detail on supported models (#17665)

## Summary

This PR removes `image_detail_original` as a runtime experiment and
makes original image detail available whenever the selected model
supports it.

Concretely, this change:
- drops the `image_detail_original` feature flag from the feature
registry and generated config schema
- makes tool-emitted image detail depend only on
`ModelInfo.supports_image_detail_original`
- updates `view_image` and `code_mode`/`js_repl` image emission to use
that capability check directly
- removes now-redundant experiment-specific tests and instruction
coverage
- keeps backward compatibility for existing configs by silently ignoring
a stale `features.image_detail_original` entry

The net effect is that `detail: "original"` is always available on
supported models, without requiring an experiment toggle.

codex-rs/features/src/lib.rs

@@ -134,8 +134,6 @@ pub enum Feature {
     Telepathy,
     /// Append additional AGENTS.md guidance to user instructions.
     ChildAgentsMd,
-    /// Allow the model to request `detail: "original"` image outputs on supported models.
-    ImageDetailOriginal,
     /// Compress request bodies (zstd) when sending streaming requests to codex-backend.
     EnableRequestCompression,
     /// Enable collab tools.
@@ -363,6 +361,9 @@ impl Features {
                 "tui_app_server" => {
                     continue;
                 }
+                "image_detail_original" => {
+                    continue;
+                }
                 _ => {}
             }
             match feature_for_key(k) {
@@ -683,16 +684,6 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::UnderDevelopment,
         default_enabled: false,
     },
-    FeatureSpec {
-        id: Feature::ImageDetailOriginal,
-        key: "image_detail_original",
-        stage: Stage::Experimental {
-            name: "Original image detail",
-            menu_description: "Let the model inspect tool-emitted images at full resolution on supported models instead of a resized approximation. This affects tool-emitted images such as those produced by `view_image`, not images attached directly in the UI. It is particularly important for localization and precise UI targeting, for reading small text, and for reasoning about precise layout.",
-            announcement: "NEW: Original image detail is now available in /experimental. Enable it to let tools request full-resolution image detail on supported models for CUA and localization tasks.",
-        },
-        default_enabled: false,
-    },
     FeatureSpec {
         id: Feature::ApplyPatchFreeform,
         key: "apply_patch_freeform",