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_tests.rs

@@ -110,6 +110,7 @@ fn test_model_client_with_thread_id(
         session_source,
         "test_originator".to_string(),
         /*model_verbosity*/ None,
+        /*content_item_kinds_enabled*/ true,
         /*enable_request_compression*/ false,
         /*include_timing_metrics*/ false,
         /*beta_features_header*/ None,
@@ -155,6 +156,7 @@ async fn compact_uses_bearer_after_agent_identity_session_fallback() -> anyhow::
         SessionSource::Cli,
         "test_originator".to_string(),
         /*model_verbosity*/ None,
+        /*content_item_kinds_enabled*/ true,
         /*enable_request_compression*/ false,
         /*include_timing_metrics*/ false,
         /*beta_features_header*/ None,
@@ -955,6 +957,7 @@ fn model_client_with_counting_attestation(
         SessionSource::Exec,
         "test_originator".to_string(),
         /*model_verbosity*/ None,
+        /*content_item_kinds_enabled*/ true,
         /*enable_request_compression*/ false,
         /*include_timing_metrics*/ false,
         /*beta_features_header*/ None,