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/core/src/tools/js_repl/mod.rs

@@ -1714,7 +1714,7 @@ fn emitted_image_content_item(
 ) -> FunctionCallOutputContentItem {
     FunctionCallOutputContentItem::InputImage {
         image_url,
-        detail: normalize_output_image_detail(turn.features.get(), &turn.model_info, detail),
+        detail: normalize_output_image_detail(&turn.model_info, detail),
     }
 }
 

codex-rs/core/src/tools/js_repl/mod_tests.rs

@@ -2,7 +2,6 @@ use super::*;
 use crate::codex::make_session_and_context;
 use crate::codex::make_session_and_context_with_dynamic_tools_and_rx;
 use crate::turn_diff_tracker::TurnDiffTracker;
-use codex_features::Feature;
 use codex_protocol::dynamic_tools::DynamicToolCallOutputContentItem;
 use codex_protocol::dynamic_tools::DynamicToolResponse;
 use codex_protocol::dynamic_tools::DynamicToolSpec;
@@ -295,42 +294,8 @@ async fn emitted_image_content_item_drops_unsupported_explicit_detail() {
 }
 
 #[tokio::test]
-async fn emitted_image_content_item_does_not_force_original_when_enabled() {
+async fn emitted_image_content_item_allows_explicit_original_detail_when_supported() {
     let (_session, mut turn) = make_session_and_context().await;
-    Arc::make_mut(&mut turn.config)
-        .features
-        .enable(Feature::ImageDetailOriginal)
-        .expect("test config should allow feature update");
-    turn.features
-        .enable(Feature::ImageDetailOriginal)
-        .expect("test turn features should allow feature update");
-    turn.model_info.supports_image_detail_original = true;
-
-    let content_item = emitted_image_content_item(
-        &turn,
-        "data:image/png;base64,AAA".to_string(),
-        /*detail*/ None,
-    );
-
-    assert_eq!(
-        content_item,
-        FunctionCallOutputContentItem::InputImage {
-            image_url: "data:image/png;base64,AAA".to_string(),
-            detail: None,
-        }
-    );
-}
-
-#[tokio::test]
-async fn emitted_image_content_item_allows_explicit_original_detail_when_enabled() {
-    let (_session, mut turn) = make_session_and_context().await;
-    Arc::make_mut(&mut turn.config)
-        .features
-        .enable(Feature::ImageDetailOriginal)
-        .expect("test config should allow feature update");
-    turn.features
-        .enable(Feature::ImageDetailOriginal)
-        .expect("test turn features should allow feature update");
     turn.model_info.supports_image_detail_original = true;
 
     let content_item = emitted_image_content_item(
@@ -349,7 +314,7 @@ async fn emitted_image_content_item_allows_explicit_original_detail_when_enabled
 }
 
 #[tokio::test]
-async fn emitted_image_content_item_drops_explicit_original_detail_when_disabled() {
+async fn emitted_image_content_item_drops_explicit_original_detail_when_unsupported() {
     let (_session, turn) = make_session_and_context().await;
 
     let content_item = emitted_image_content_item(