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/features/src/lib.rs

@@ -99,6 +99,8 @@ pub enum Feature {
     SecretAuthStorage,
 
     // Experimental
+    /// Send per-content-entry classifications in internal Responses metadata.
+    ContentItemKinds,
     /// Record model-attempted tool calls in internal Responses metadata.
     ExecutedToolCallMetadata,
     /// Enable JavaScript code mode backed by the standalone host process.
@@ -905,6 +907,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Removed,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::ContentItemKinds,
+        key: "content_item_kinds",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::ExecutedToolCallMetadata,
         key: "executed_tool_call_metadata",