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/src/session/session.rs

@@ -1432,6 +1432,7 @@ impl Session {
                     session_configuration.session_source.clone(),
                     session_configuration.originator.clone(),
                     config.model_verbosity,
+                    config.features.enabled(Feature::ContentItemKinds),
                     config.features.enabled(Feature::EnableRequestCompression),
                     config.features.enabled(Feature::RuntimeMetrics),
                     Self::build_model_client_beta_features_header(config.as_ref()),