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/client.rs

@@ -208,6 +208,7 @@ struct ModelClientState {
     session_source: SessionSource,
     originator: String,
     model_verbosity: Option<VerbosityConfig>,
+    content_item_kinds_enabled: bool,
     enable_request_compression: bool,
     include_timing_metrics: bool,
     beta_features_header: Option<String>,
@@ -436,6 +437,7 @@ impl ModelClient {
         session_source: SessionSource,
         originator: String,
         model_verbosity: Option<VerbosityConfig>,
+        content_item_kinds_enabled: bool,
         enable_request_compression: bool,
         include_timing_metrics: bool,
         beta_features_header: Option<String>,
@@ -459,6 +461,7 @@ impl ModelClient {
                 session_source,
                 originator,
                 model_verbosity,
+                content_item_kinds_enabled,
                 enable_request_compression,
                 include_timing_metrics,
                 beta_features_header,
@@ -963,6 +966,9 @@ impl ModelClient {
             if item.id().is_some_and(|id| !id.is_prefixed()) {
                 item.set_id(/*new_id*/ None);
             }
+            if !self.state.content_item_kinds_enabled {
+                item.clear_content_item_kinds();
+            }
         }
     }