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/config.schema.json

@@ -398,9 +398,6 @@
             "guardian_approval": {
               "type": "boolean"
             },
-            "image_detail_original": {
-              "type": "boolean"
-            },
             "image_generation": {
               "type": "boolean"
             },
@@ -2254,9 +2251,6 @@
         "guardian_approval": {
           "type": "boolean"
         },
-        "image_detail_original": {
-          "type": "boolean"
-        },
         "image_generation": {
           "type": "boolean"
         },