refactor: 拆分 scheduler 为 TaskManager + Executor

- InferenceScheduler 退化为编排器，委托 TaskManager 管理任务生命周期 + Executor 执行模型前向
- Task/TaskStatus/TaskManager 移至 task.py
- Executor 移至 executor.py (原 BatchExecutor)
- scheduler.py 437 行 -> 142 行

astrai/inference/__init__.py

@@ -21,11 +21,8 @@ from astrai.inference.sampling import (
     TopPStrategy,
     sample,
 )
-from astrai.inference.scheduler import (
-    InferenceScheduler,
-    Task,
-    TaskStatus,
-)
+from astrai.inference.scheduler import InferenceScheduler
+from astrai.inference.task import Task, TaskStatus
 
 __all__ = [
     # Engine / Requests