refactor: 修复候选分页逻辑并拆分配额检查模块

- list_all_candidates 返回 provider_batch_count，区分"无候选"与"无 Provider"
  避免分页在有 Provider 但无候选时提前终止
- 将配额检查逻辑从 aware_scheduler.py 拆分到 quota_skipper.py
- 提取 reorder_candidates 方法，支持跨页汇总后全局重排序
- CandidateResolver 分页循环改用 provider_batch_count 判断终止
- candidate extra_data 中新增 mapping_matched_model 字段
- 新增契约测试和分页行为测试

tests/services/test_candidate_resolver_pagination.py

@@ -0,0 +1,97 @@
+from __future__ import annotations
+
+from types import SimpleNamespace
+from typing import Any
+from unittest.mock import MagicMock
+
+import pytest
+
+from src.services.cache.aware_scheduler import CacheAwareScheduler
+from src.services.orchestration.candidate_resolver import CandidateResolver
+
+
+class _FakeScheduler:
+    def __init__(self) -> None:
+        self.calls: list[int] = []
+        self.scheduling_mode = CacheAwareScheduler.SCHEDULING_MODE_FIXED_ORDER
+        self.priority_mode = CacheAwareScheduler.PRIORITY_MODE_PROVIDER
+
+    async def list_all_candidates(
+        self,
+        *,
+        db: Any,
+        api_format: str,
+        model_name: str,
+        affinity_key: str | None = None,
+        user_api_key: Any | None = None,
+        provider_offset: int = 0,
+        provider_limit: int | None = None,
+        max_candidates: int | None = None,
+        is_stream: bool = False,
+        capability_requirements: dict[str, bool] | None = None,
+    ) -> tuple[list[Any], str, int]:
+        _ = (
+            db,
+            api_format,
+            model_name,
+            affinity_key,
+            user_api_key,
+            max_candidates,
+            is_stream,
+            capability_requirements,
+        )
+        assert provider_limit is not None
+
+        self.calls.append(int(provider_offset))
+
+        if provider_offset == 0:
+            # Simulate a provider page that has providers but no eligible candidates.
+            return [], "gm1", int(provider_limit)
+
+        if provider_offset == int(provider_limit):
+            # Next page yields one eligible candidate and is also the last provider page.
+            cand = SimpleNamespace(
+                provider=SimpleNamespace(id="p1", name="prov"),
+                endpoint=SimpleNamespace(id="e1"),
+                key=SimpleNamespace(id="k1"),
+                is_skipped=False,
+                skip_reason=None,
+                is_cached=False,
+                needs_conversion=False,
+                provider_api_format=str(api_format),
+                mapping_matched_model=None,
+            )
+            return [cand], "gm1", 5
+
+        return [], "gm1", 0
+
+    async def reorder_candidates(
+        self,
+        candidates: list[Any],
+        db: Any = None,
+        affinity_key: str | None = None,
+        api_format: str | None = None,
+        global_model_id: str | None = None,
+    ) -> list[Any]:
+        return candidates
+
+
+@pytest.mark.asyncio
+async def test_candidate_resolver_pagination_continues_on_empty_candidate_batch() -> None:
+    db = MagicMock()
+    scheduler = _FakeScheduler()
+    resolver = CandidateResolver(db=db, cache_scheduler=scheduler)  # type: ignore[arg-type]
+
+    candidates, global_model_id = await resolver.fetch_candidates(
+        api_format="openai:chat",
+        model_name="gpt-4o",
+        affinity_key="a1",
+        user_api_key=None,
+        request_id="r1",
+        is_stream=False,
+        capability_requirements=None,
+    )
+
+    assert global_model_id == "gm1"
+    assert len(candidates) == 1
+    assert scheduler.calls == [0, 20]