feat(pool): 号池候选重构为 PoolCandidate 单候选模式与池内 key 故障转移

- 新增 PoolCandidate 子类，排序阶段作为单候选参与，执行阶段在 pool_keys 内部选择/切换 key
- FailoverEngine 新增 _execute_pool_candidate 方法，支持池内 key 级别故障转移与重试
- 提取 _execute_attempt / _attach_attempt_context / _classify_attempt_error 公共方法
- CandidateBuilder 对号池 Provider 构建单个 PoolCandidate（包含所有可用 key）
- CandidateSorter 支持 PoolCandidate 独立优先级分组（global_priority / pool_priority）
- CandidateResolver PRE_EXPAND 模式按 pool_keys 展开预创建记录，附加 pool_group_id
- TaskService._apply_pool_reorder 改为对 PoolCandidate 调用 select_pool_keys
- PoolManager 新增 select_pool_keys 方法，复用 reorder_candidates 逻辑
- 新增 global_priority 号池配置字段（前后端同步）
- 前端 Timeline 支持按 pool_group_id 分组显示多号池尝试
- 异步提交路径新增 _expand_pool_candidates_for_async_submit 展开逻辑

tests/services/test_aware_scheduler_pool_candidate_enumeration.py

@@ -3,6 +3,7 @@ from unittest.mock import AsyncMock, MagicMock
 import pytest
 
 from src.services.scheduling.aware_scheduler import CacheAwareScheduler
+from src.services.scheduling.schemas import PoolCandidate
 
 
 def _mock_key(key_id: str, api_formats: list[str]) -> MagicMock:
@@ -27,7 +28,7 @@ def _mock_endpoint(api_format: str) -> MagicMock:
 
 
 @pytest.mark.asyncio
-async def test_pool_provider_enumerates_all_key_candidates() -> None:
+async def test_pool_provider_builds_single_pool_candidate() -> None:
     scheduler = CacheAwareScheduler()
     builder = scheduler._candidate_builder
     builder._check_model_support = AsyncMock(return_value=(True, None, None, {"m"}))  # type: ignore[method-assign]
@@ -53,5 +54,8 @@ async def test_pool_provider_enumerates_all_key_candidates() -> None:
         global_conversion_enabled=True,
     )
 
-    assert len(candidates) == 2
-    assert {str(c.key.id) for c in candidates} == {"k1", "k2"}
+    assert len(candidates) == 1
+    pool_candidate = candidates[0]
+    assert isinstance(pool_candidate, PoolCandidate)
+    assert str(pool_candidate.key.id) == "k1"
+    assert {str(k.id) for k in pool_candidate.pool_keys} == {"k1", "k2"}