revert: restore async for in handle_completed_job

Modal environment requires async for Sandbox.list(). The
TypeError seen earlier was likely a transient Modal runtime
issue, not a sync/async mismatch. Reverting my incorrect fix.

Author: manascb1344

runner/services/job_service.py

@@ -355,7 +355,7 @@ async def handle_completed_job(app: modal.App, payload: dict) -> dict:
     conclusion = workflow_job.get("conclusion", "")
 
     if conclusion == "cancelled":
-        for sb in modal.Sandbox.list(app_id=app.app_id, tags={"job_id": job_id}):
+        async for sb in modal.Sandbox.list(app_id=app.app_id, tags={"job_id": job_id}):
             if sb.poll() is None:
                 logger.info("Terminating sandbox for cancelled job", extra={"job_id": job_id})
                 try: