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/tests/common/test_codex.rs

@@ -1339,6 +1339,10 @@ 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![],