fix: 流式响应健壮性增强与候选排序稳定性修复

- test-model 接口优先使用流式请求，失败自动回退非流式
- CLI 流式处理在 StreamClosed/RemoteProtocolError 时 flush 残余 SSE 数据，捕获尾部 usage
- 流未正常完成时兜底估算 tokens，避免 usage 记录为 0
- ProviderCandidate 添加 __lt__ 解决 tuple 排序 TypeError
- sorted() 添加显式 key 参数避免隐式比较候选对象
- 异常日志改用 logger.opt(exception=e) 替代手动 traceback

tests/unit/test_provider_candidate_ordering.py

@@ -0,0 +1,81 @@
+from __future__ import annotations
+
+from types import SimpleNamespace
+
+from src.services.cache.aware_scheduler import ProviderCandidate
+
+
+def _make_candidate(
+    *,
+    provider_priority: int | None,
+    internal_priority: int | None,
+    provider_id: str,
+    endpoint_id: str,
+    key_id: str,
+) -> ProviderCandidate:
+    provider = SimpleNamespace(id=provider_id, name="p", provider_priority=provider_priority)
+    endpoint = SimpleNamespace(id=endpoint_id)
+    key = SimpleNamespace(id=key_id, internal_priority=internal_priority)
+    return ProviderCandidate(provider=provider, endpoint=endpoint, key=key)  # type: ignore[arg-type]
+
+
+def test_provider_candidate_is_orderable() -> None:
+    c1 = _make_candidate(
+        provider_priority=1,
+        internal_priority=1,
+        provider_id="p1",
+        endpoint_id="e1",
+        key_id="k1",
+    )
+    c2 = _make_candidate(
+        provider_priority=1,
+        internal_priority=2,
+        provider_id="p1",
+        endpoint_id="e1",
+        key_id="k2",
+    )
+
+    assert c1 < c2
+    assert sorted([c2, c1]) == [c1, c2]
+
+
+def test_provider_candidate_can_break_ties_in_tuple_sort() -> None:
+    c1 = _make_candidate(
+        provider_priority=1,
+        internal_priority=1,
+        provider_id="p1",
+        endpoint_id="e1",
+        key_id="k1",
+    )
+    c2 = _make_candidate(
+        provider_priority=1,
+        internal_priority=2,
+        provider_id="p1",
+        endpoint_id="e1",
+        key_id="k2",
+    )
+
+    # When the first tuple element ties, Python will compare the second element.
+    # This should not raise:
+    # TypeError: '<' not supported between instances of 'ProviderCandidate' and 'ProviderCandidate'
+    pairs = [(0, c2), (0, c1)]
+    assert [c for _score, c in sorted(pairs)] == [c1, c2]
+
+
+def test_provider_candidate_none_priorities_sort_last() -> None:
+    c1 = _make_candidate(
+        provider_priority=1,
+        internal_priority=None,
+        provider_id="p1",
+        endpoint_id="e1",
+        key_id="k1",
+    )
+    c2 = _make_candidate(
+        provider_priority=None,
+        internal_priority=None,
+        provider_id="p2",
+        endpoint_id="e1",
+        key_id="k2",
+    )
+
+    assert sorted([c2, c1]) == [c1, c2]