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/cli/src/main.rs

@@ -4133,6 +4133,16 @@ mod tests {
         assert_eq!(overrides, vec!["features.enable_fanout=true".to_string(),]);
     }
 
+    #[test]
+    fn feature_toggles_accept_removed_item_ids_flag() {
+        let toggles = FeatureToggles {
+            enable: vec!["item_ids".to_string()],
+            disable: Vec::new(),
+        };
+        let overrides = toggles.to_overrides().expect("valid features");
+        assert_eq!(overrides, vec!["features.item_ids=true".to_string()]);
+    }
+
     #[test]
     fn feature_toggles_unknown_feature_errors() {
         let toggles = FeatureToggles {