fix(runtime): make rollback restore checkpoint supersede newer checkpoints (#2582)

* Restore rollback checkpoints with fresh ids

* Tighten rollback checkpoint tests and imports

* Update test_run_worker_rollback.py

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

backend/app/gateway/routers/threads.py

@@ -18,6 +18,7 @@ import uuid
 from typing import Any
 
 from fastapi import APIRouter, HTTPException, Request
+from langgraph.checkpoint.base import empty_checkpoint
 from pydantic import BaseModel, Field, field_validator
 
 from app.gateway.authz import require_permission
@@ -262,8 +263,6 @@ async def create_thread(body: ThreadCreateRequest, request: Request) -> ThreadRe
     # Write an empty checkpoint so state endpoints work immediately
     config = {"configurable": {"thread_id": thread_id, "checkpoint_ns": ""}}
     try:
-        from langgraph.checkpoint.base import empty_checkpoint
-
         ckpt_metadata = {
             "step": -1,
             "source": "input",