refactor: 统一任务框架 Phase 3 - 用 TaskService/FailoverEngine 替代 FallbackOrchestrator

核心重构：
- 移除 FallbackOrchestrator，用 TaskService + FailoverEngine 替代
- TaskService 作为统一入口，支持 SYNC/ASYNC 两种任务模式
- FailoverEngine 实现候选遍历、重试、故障转移逻辑
- 新增 AttemptFunc/AttemptResult 协议，统一尝试结果表示

功能改进：
- 流式响应首字节探测（30s 超时，空流触发故障转移）
- 流式取消归因优化（区分客户端断连 vs 服务端中断）
- 新增 OpenAI Sora 视频取消路由 POST /v1/videos/{task_id}/cancel
- OpenAI 流式请求自动添加 stream_options.include_usage

代码规范：
- 修复 loguru 日志格式（%s → {}）
- 新增 FORMAT_CONVERSION_ENABLED 环境变量说明

测试覆盖：
- test_failover_engine.py: FailoverEngine 单元测试
- test_task_service_async_execute.py: TaskService ASYNC 模式测试
- test_video_cancel_e2e.py: 视频取消端到端测试

tests/services/test_task_service_async_execute.py

@@ -0,0 +1,74 @@
+from __future__ import annotations
+
+from types import SimpleNamespace
+from unittest.mock import AsyncMock, MagicMock
+
+import pytest
+
+from src.services.candidate.schema import CandidateKey
+from src.services.candidate.submit import SubmitOutcome
+from src.services.task.context import TaskMode
+from src.services.task.protocol import AttemptKind
+from src.services.task.service import TaskService
+
+
+@pytest.mark.asyncio
+async def test_task_service_execute_async_requires_extract_external_task_id() -> None:
+    svc = TaskService(MagicMock())
+    with pytest.raises(ValueError):
+        await svc.execute(
+            task_type="video",
+            task_mode=TaskMode.ASYNC,
+            api_format="openai:video",
+            model_name="m",
+            user_api_key=MagicMock(id="u", user_id="user"),
+            request_func=AsyncMock(),
+            request_id="rid",
+        )
+
+
+@pytest.mark.asyncio
+async def test_task_service_execute_async_returns_execution_result() -> None:
+    db = MagicMock()
+    svc = TaskService(db)
+
+    candidate = SimpleNamespace(
+        provider=SimpleNamespace(id="p1", name="prov"),
+        endpoint=SimpleNamespace(id="e1"),
+        key=SimpleNamespace(id="k1"),
+    )
+    outcome = SubmitOutcome(
+        candidate=candidate,
+        candidate_keys=[{"index": 0, "provider_id": "p1"}],
+        external_task_id="task_123",
+        rule_lookup=None,
+        upstream_payload={"id": "x"},
+        upstream_headers={"x-test": "1"},
+        upstream_status_code=200,
+    )
+
+    svc.submit_with_failover = AsyncMock(return_value=outcome)  # type: ignore[method-assign]
+    svc._recorder.get_candidate_keys = MagicMock(  # type: ignore[attr-defined]
+        return_value=[
+            CandidateKey(candidate_index=0, retry_index=0, status="success", provider_id="p1")
+        ]
+    )
+
+    result = await svc.execute(
+        task_type="video",
+        task_mode=TaskMode.ASYNC,
+        api_format="openai:video",
+        model_name="m",
+        user_api_key=MagicMock(id="u", user_id="user"),
+        request_func=AsyncMock(),
+        request_id="rid",
+        extract_external_task_id=MagicMock(),
+        allow_format_conversion=True,
+    )
+
+    assert result.success is True
+    assert result.attempt_result is not None
+    assert result.attempt_result.kind == AttemptKind.ASYNC_SUBMIT
+    assert result.provider_task_id == "task_123"
+    assert result.provider_id == "p1"
+    assert result.candidate_index == 0