refactor : BaseConfig 提供 from_json/to_json，嵌套 config 自动反序列化

- from_json/to_json 上提至 BaseConfig，所有子类自动继承
- _coerce 新增 dict 到 BaseConfig 子类的递归反序列化，消除子类 from_dict 重载
- PipelineConfig 等子类仅声明字段，零样板代码
- 测试 tokenizer 改为自包含 BPE（含 chat template），不依赖 params/ 目录
- 特殊 token 改用 ASCII 字符，兼容所有平台

astrai/config/base.py

@@ -1,6 +1,7 @@
 import json
 from dataclasses import MISSING, dataclass, fields
-from typing import Any, Dict, Optional, Self, get_type_hints
+from pathlib import Path
+from typing import Any, Dict, Optional, Self, Union, get_type_hints
 
 
 @dataclass
@@ -83,4 +84,15 @@ class BaseConfig:
             return value
         if isinstance(value, target_type):
             return value
+        if isinstance(value, dict) and issubclass(target_type, BaseConfig):
+            return target_type.from_dict(value)
         raise TypeError
+
+    @classmethod
+    def from_json(cls, path: Union[str, Path]) -> Self:
+        with open(path, "r", encoding="utf-8") as f:
+            return cls.from_dict(json.load(f))
+
+    def to_json(self, path: Union[str, Path]):
+        with open(path, "w", encoding="utf-8") as f:
+            json.dump(self.to_dict(), f, indent=2, ensure_ascii=False)