refactor: 消除多处重复模式，统一工厂和参数传递

- AutoModel 继承 BaseFactory，消除自建 Registry（-30 行）
- executor.execute_prefill 删除重复 forward 代码块（bug）
- train_callback 移除 Protocol 上矛盾的 issubclass 检查
- engine.py 内部方法统一传 GenerationParams，校验内聚
- protocol.py SSEBuilder 类→函数，handle() 用 GenerationParams
- StreamContext 动态属性改为显式 dataclass 字段
- BaseFactory 新增 get_component_class 方法

astrai/factory.py

@@ -155,6 +155,26 @@ class BaseFactory(ABC, Generic[T]):
         """
         pass
 
+    @classmethod
+    def get_component_class(cls, name: str) -> Type[T]:
+        """Get the registered component class by name without instantiating it.
+
+        Args:
+            name: Registered name of the component
+
+        Returns:
+            The component class itself
+
+        Raises:
+            ValueError: If the component name is not registered
+        """
+        if not cls._registry.contains(name):
+            raise ValueError(
+                f"Unknown component: '{name}'. "
+                f"Supported types: {sorted(cls._registry.list_names())}"
+            )
+        return cls._registry.get(name)
+
     @classmethod
     def list_registered(cls) -> list:
         """List all registered component names.