fix: ProgressBar默认输出到stdout

- file参数默认值改为None, 内部用 or sys.stdout 兜底
- 清理inference API中未使用的import (Optional, time, field)
- 删除test_protocol中未使用的ctx变量

astrai/inference/api/protocol.py

@@ -5,9 +5,8 @@ protocol-specific formatting to a ResponseBuilder.
 """
 
 import json
-import time
 from abc import ABC, abstractmethod
-from dataclasses import dataclass, field
+from dataclasses import dataclass
 from typing import Any, AsyncGenerator, Dict, List, Optional, Tuple, Union
 
 from fastapi.responses import StreamingResponse