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?;