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/memories/write/src/runtime.rs

@@ -257,6 +257,7 @@ impl MemoryStartupContext {
             session_source.clone(),
             config_snapshot.originator,
             config.model_verbosity,
+            config.features.enabled(Feature::ContentItemKinds),
             config.features.enabled(Feature::EnableRequestCompression),
             config.features.enabled(Feature::RuntimeMetrics),
             /*beta_features_header*/ None,