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/tests.rs

@@ -169,18 +169,6 @@ fn use_agent_identity_is_under_development() {
     assert_eq!(Feature::UseAgentIdentity.default_enabled(), false);
 }
 
-#[test]
-fn image_detail_original_feature_is_experimental_and_user_toggleable() {
-    let stage = Feature::ImageDetailOriginal.stage();
-
-    assert!(matches!(stage, Stage::Experimental { .. }));
-    assert_eq!(
-        stage.experimental_menu_name(),
-        Some("Original image detail")
-    );
-    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));
@@ -263,6 +251,25 @@ fn from_sources_applies_base_profile_and_overrides() {
     assert_eq!(features.enabled(Feature::WebSearchRequest), false);
 }
 
+#[test]
+fn from_sources_ignores_removed_image_detail_original_feature_key() {
+    let features_toml = FeaturesToml::from(BTreeMap::from([(
+        "image_detail_original".to_string(),
+        true,
+    )]));
+
+    let features = Features::from_sources(
+        FeatureConfigSource {
+            features: Some(&features_toml),
+            ..Default::default()
+        },
+        FeatureConfigSource::default(),
+        FeatureOverrides::default(),
+    );
+
+    assert_eq!(features, Features::with_defaults());
+}
+
 #[test]
 fn multi_agent_v2_feature_config_deserializes_boolean_toggle() {
     let features: FeaturesToml = toml::from_str(