fix(scheduling): 负载均衡模式与无亲和键场景统一走随机排序

重构 CandidateSorter.shuffle_keys_by_internal_priority 中同优先级
Key 的排序逻辑，将三分支简化为两分支：
- 随机排序：TTL=0 / 负载均衡模式 / 无 affinity_key
- 哈希确定性排序：缓存亲和模式且有 affinity_key

移除了原先"无 affinity_key 时按 ID 排序"的冗余分支，新增
对应单元测试覆盖三种场景。

tests/unit/test_candidate_sorter_key_shuffle.py

@@ -0,0 +1,93 @@
+from __future__ import annotations
+
+from types import SimpleNamespace
+from typing import cast
+from unittest.mock import patch
+
+from src.models.database import ProviderAPIKey
+from src.services.scheduling.candidate_sorter import CandidateSorter
+from src.services.scheduling.scheduling_config import SchedulingConfig
+from src.services.scheduling.utils import affinity_hash
+
+
+def _make_key(key_id: str, internal_priority: int = 1) -> ProviderAPIKey:
+    return cast(
+        ProviderAPIKey,
+        SimpleNamespace(
+            id=key_id,
+            internal_priority=internal_priority,
+        ),
+    )
+
+
+def _reverse_in_place(items: list[object]) -> None:
+    items.reverse()
+
+
+def test_shuffle_keys_random_in_load_balance_mode_even_with_affinity_key() -> None:
+    config = SchedulingConfig(
+        priority_mode=SchedulingConfig.PRIORITY_MODE_PROVIDER,
+        scheduling_mode=SchedulingConfig.SCHEDULING_MODE_LOAD_BALANCE,
+    )
+    sorter = CandidateSorter(config)
+
+    keys = [_make_key("k1"), _make_key("k2"), _make_key("k3")]
+
+    with patch(
+        "src.services.scheduling.candidate_sorter.random.shuffle",
+        side_effect=_reverse_in_place,
+    ) as shuffle_mock:
+        result = sorter.shuffle_keys_by_internal_priority(
+            keys,
+            affinity_key="affinity-1",
+            use_random=False,
+        )
+
+    assert [k.id for k in result] == ["k3", "k2", "k1"]
+    assert shuffle_mock.call_count == 1
+
+
+def test_shuffle_keys_random_when_affinity_key_absent() -> None:
+    config = SchedulingConfig(
+        priority_mode=SchedulingConfig.PRIORITY_MODE_PROVIDER,
+        scheduling_mode=SchedulingConfig.SCHEDULING_MODE_CACHE_AFFINITY,
+    )
+    sorter = CandidateSorter(config)
+
+    keys = [_make_key("k1"), _make_key("k2"), _make_key("k3")]
+
+    with patch(
+        "src.services.scheduling.candidate_sorter.random.shuffle",
+        side_effect=_reverse_in_place,
+    ) as shuffle_mock:
+        result = sorter.shuffle_keys_by_internal_priority(
+            keys,
+            affinity_key=None,
+            use_random=False,
+        )
+
+    assert [k.id for k in result] == ["k3", "k2", "k1"]
+    assert shuffle_mock.call_count == 1
+
+
+def test_shuffle_keys_still_hashes_with_affinity_in_non_load_balance_mode() -> None:
+    config = SchedulingConfig(
+        priority_mode=SchedulingConfig.PRIORITY_MODE_PROVIDER,
+        scheduling_mode=SchedulingConfig.SCHEDULING_MODE_CACHE_AFFINITY,
+    )
+    sorter = CandidateSorter(config)
+
+    keys = [_make_key("k1"), _make_key("k2"), _make_key("k3")]
+    affinity_key = "affinity-1"
+
+    expected = sorted(keys, key=lambda k: affinity_hash(affinity_key, k.id))
+
+    with patch("src.services.scheduling.candidate_sorter.random.shuffle") as shuffle_mock:
+        result = sorter.shuffle_keys_by_internal_priority(
+            keys,
+            affinity_key=affinity_key,
+            use_random=False,
+        )
+
+    assert [k.id for k in result] == [k.id for k in expected]
+    shuffle_mock.assert_not_called()