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/core/src/compact_remote_v2.rs

@@ -300,7 +300,6 @@ async fn run_remote_compact_task_inner_impl(
         });
     }
     sess.replace_compacted_history(
-        compaction_turn_context.as_ref(),
         new_history,
         reference_context_item,
         world_state_baseline,