Gate content item kinds behind a feature flag (#40450)

## What changed

- Add the under-development `content_item_kinds` feature, disabled by default.
- Strip content item classifications from Responses input when the feature is
  disabled while preserving other passthrough metadata.

## Testing

- Add coverage that disables the feature and verifies requests omit content item
  kinds without removing the turn ID.

GitOrigin-RevId: f938feb2895d766d979b1666fc7b97fd128fe8a7

codex-rs/core/config.schema.json

@@ -652,6 +652,9 @@
             "connectors": {
               "type": "boolean"
             },
+            "content_item_kinds": {
+              "type": "boolean"
+            },
             "current_time_reminder": {
               "$ref": "#/definitions/FeatureToml_for_CurrentTimeReminderConfigToml"
             },
@@ -5628,6 +5631,9 @@
         "connectors": {
           "type": "boolean"
         },
+        "content_item_kinds": {
+          "type": "boolean"
+        },
         "current_time_reminder": {
           "$ref": "#/definitions/FeatureToml_for_CurrentTimeReminderConfigToml"
         },