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/tools/src/tool_config.rs

@@ -153,7 +153,7 @@ impl ToolsConfig {
         let include_tool_suggest = features.enabled(Feature::ToolSuggest)
             && features.enabled(Feature::Apps)
             && features.enabled(Feature::Plugins);
-        let include_original_image_detail = can_request_original_image_detail(features, model_info);
+        let include_original_image_detail = can_request_original_image_detail(model_info);
         // API-key auth bypasses Codex backend entitlement/tool normalization, so
         // callers must confirm ChatGPT auth before exposing the built-in tool.
         let include_image_gen_tool = *image_generation_tool_auth_allowed