Always assign response item IDs (#34645)

## What changed

- Assign IDs to client-created response items in every session, including streamed items, forked history, compaction results, and non-OpenAI provider requests.
- Preserve server-provided IDs and stable generated IDs across persisted and resumed conversations while continuing to omit unprefixed server IDs from outbound requests.
- Retire `features.item_ids` as a configurable feature. Existing configuration and CLI toggles remain accepted as compatibility no-ops.

## Testing

- Cover default ID assignment, persistence and resume behavior, remote compaction, provider requests, WebSocket requests, and removed-feature compatibility.

GitOrigin-RevId: 7a09e2d07d2606c2ef1e8e2283d36cb555557eae

codex-rs/features/src/tests.rs

@@ -274,6 +274,23 @@ fn from_sources_ignores_removed_resize_all_images_feature_key() {
     assert_eq!(features, Features::with_defaults());
 }
 
+#[test]
+fn from_sources_ignores_removed_item_ids_feature_key() {
+    let features_toml = FeaturesToml::from(BTreeMap::from([("item_ids".to_string(), false)]));
+
+    let features = Features::from_sources(
+        FeatureConfigSource {
+            features: Some(&features_toml),
+            ..Default::default()
+        },
+        FeatureConfigSource::default(),
+        FeatureOverrides::default(),
+    );
+
+    assert_eq!(features, Features::with_defaults());
+    assert_eq!(features.enabled(Feature::ItemIds), true);
+}
+
 #[test]
 fn from_sources_ignores_removed_undo_feature_key() {
     let features_toml = FeaturesToml::from(BTreeMap::from([("undo".to_string(), true)]));