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/tests/suite/code_mode.rs

@@ -1832,7 +1832,6 @@ async fn code_mode_can_use_view_image_result_with_image_helper() -> Result<()> {
         .with_model("gpt-5.3-codex")
         .with_config(move |config| {
             let _ = config.features.enable(Feature::CodeMode);
-            let _ = config.features.enable(Feature::ImageDetailOriginal);
         });
     let test = builder.build(&server).await?;
 

codex-rs/core/tests/suite/view_image.rs

@@ -3,7 +3,6 @@
 use base64::Engine;
 use base64::engine::general_purpose::STANDARD as BASE64_STANDARD;
 use codex_exec_server::CreateDirectoryOptions;
-use codex_features::Feature;
 use codex_login::CodexAuth;
 use codex_protocol::config_types::ReasoningSummary;
 use codex_protocol::openai_models::ConfigShellToolType;
@@ -361,14 +360,7 @@ async fn view_image_tool_can_preserve_original_resolution_when_requested_on_gpt5
     skip_if_no_network!(Ok(()));
 
     let server = start_mock_server().await;
-    let mut builder = test_codex()
-        .with_model("gpt-5.3-codex")
-        .with_config(|config| {
-            config
-                .features
-                .enable(Feature::ImageDetailOriginal)
-                .expect("test config should allow feature update");
-        });
+    let mut builder = test_codex().with_model("gpt-5.3-codex");
     let test = builder.build_remote_aware(&server).await?;
     let TestCodex {
         codex,
@@ -469,14 +461,7 @@ async fn view_image_tool_errors_clearly_for_unsupported_detail_values() -> anyho
     skip_if_no_network!(Ok(()));
 
     let server = start_mock_server().await;
-    let mut builder = test_codex()
-        .with_model("gpt-5.3-codex")
-        .with_config(|config| {
-            config
-                .features
-                .enable(Feature::ImageDetailOriginal)
-                .expect("test config should allow feature update");
-        });
+    let mut builder = test_codex().with_model("gpt-5.3-codex");
     let test = builder.build_remote_aware(&server).await?;
     let TestCodex {
         codex,
@@ -559,14 +544,7 @@ async fn view_image_tool_treats_null_detail_as_omitted() -> anyhow::Result<()> {
     skip_if_no_network!(Ok(()));
 
     let server = start_mock_server().await;
-    let mut builder = test_codex()
-        .with_model("gpt-5.3-codex")
-        .with_config(|config| {
-            config
-                .features
-                .enable(Feature::ImageDetailOriginal)
-                .expect("test config should allow feature update");
-        });
+    let mut builder = test_codex().with_model("gpt-5.3-codex");
     let test = builder.build_remote_aware(&server).await?;
     let TestCodex {
         codex,
@@ -661,12 +639,7 @@ async fn view_image_tool_resizes_when_model_lacks_original_detail_support() -> a
     skip_if_no_network!(Ok(()));
 
     let server = start_mock_server().await;
-    let mut builder = test_codex().with_model("gpt-5.2").with_config(|config| {
-        config
-            .features
-            .enable(Feature::ImageDetailOriginal)
-            .expect("test config should allow feature update");
-    });
+    let mut builder = test_codex().with_model("gpt-5.2");
     let test = builder.build_remote_aware(&server).await?;
     let TestCodex {
         codex,
@@ -765,19 +738,12 @@ async fn view_image_tool_resizes_when_model_lacks_original_detail_support() -> a
 }
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
-async fn view_image_tool_does_not_force_original_resolution_with_capability_feature_only()
+async fn view_image_tool_does_not_force_original_resolution_with_capability_only()
 -> anyhow::Result<()> {
     skip_if_no_network!(Ok(()));
 
     let server = start_mock_server().await;
-    let mut builder = test_codex()
-        .with_model("gpt-5.3-codex")
-        .with_config(|config| {
-            config
-                .features
-                .enable(Feature::ImageDetailOriginal)
-                .expect("test config should allow feature update");
-        });
+    let mut builder = test_codex().with_model("gpt-5.3-codex");
     let test = builder.build_remote_aware(&server).await?;
     let TestCodex {
         codex,
@@ -1533,3 +1499,4 @@ async fn replaces_invalid_local_image_after_bad_request() -> anyhow::Result<()>
 
     Ok(())
 }
+use codex_features::Feature;