fix(ui): guard reasoning renderer against undefined text (#28222)

packages/ui/src/components/message-part.tsx

@@ -1563,7 +1563,7 @@ PART_MAPPING["reasoning"] = function ReasoningPartDisplay(props) {
   const streaming = createMemo(
     () => props.message.role === "assistant" && typeof (props.message as AssistantMessage).time.completed !== "number",
   )
-  const text = () => (data.store.part_text_accum_delta?.[part().id] ?? part().text).trim()
+  const text = () => (data.store.part_text_accum_delta?.[part().id] ?? part().text ?? "").trim()
 
   return (
     <Show when={text()}>