Enable content item kinds by default (#40669)

## What changed

Mark `content_item_kinds` as stable and enable it by default. Remove the
test harness override that previously enabled the feature explicitly.

GitOrigin-RevId: 86b0c3f295c4de3b8900981bfdd8830f275073f5

codex-rs/features/src/lib.rs

@@ -922,8 +922,8 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::ContentItemKinds,
         key: "content_item_kinds",
-        stage: Stage::UnderDevelopment,
-        default_enabled: false,
+        stage: Stage::Stable,
+        default_enabled: true,
     },
     FeatureSpec {
         id: Feature::ExecutedToolCallMetadata,