refactor(task): 拆分 TaskService 并重构任务生命周期

- 将 task 公共协议/上下文/异常/schema 下沉到 core，并迁移 polling 目录
- 新增 execute/submit/video 子模块，拆分同步执行、异步提交流程、错误处理与视频任务操作
- 收敛 TaskService 为门面编排，内部委派到 SyncTaskExecutionService、AsyncTaskSubmitService、VideoTaskOperationsService
- 重构 main 生命周期管理：引入 LifecycleState，拆分启动与关闭流程
- 删除未使用的 TaskExecuteFacadeService 与 TaskSubmitFacadeService

Closes #201

Co-authored-by: AAEE86 <ppk0227@hotmail.com>

src/api/admin/provider_query.py

@@ -1956,11 +1956,14 @@ async def test_model_failover(
     - direct: 直接测试 provider_model_name，在当前 Provider 内多 Key 故障转移
     """
     from src.core.exceptions import ProviderNotAvailableException
+    from src.services.candidate.failover import FailoverEngine
+    from src.services.candidate.policy import RetryMode, RetryPolicy, SkipPolicy
     from src.services.candidate.recorder import CandidateRecorder
     from src.services.scheduling.candidate_builder import CandidateBuilder
     from src.services.scheduling.candidate_sorter import CandidateSorter
     from src.services.scheduling.scheduling_config import SchedulingConfig
     from src.services.task import TaskService
+    from src.services.task.core.protocol import AttemptKind, AttemptResult
 
     provider = (
         db.query(Provider)