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/core/tests/common/test_codex.rs

@@ -1339,10 +1339,6 @@ pub fn test_codex() -> TestCodexBuilder {
                 .features
                 .disable(Feature::ShellSnapshot)
                 .expect("test config should allow ShellSnapshot override");
-            config
-                .features
-                .enable(Feature::ContentItemKinds)
-                .expect("test config should allow ContentItemKinds override");
         })],
         auth: CodexAuth::from_api_key("dummy"),
         pre_build_hooks: vec![],

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,