fix: harden the observability fields against the verifier findings

tests/unit/test_pricing.py

@@ -77,6 +77,10 @@ class TestCachedInputTier:
     def test_no_hit_is_billed_in_full(self, cached):
         assert self._CACHED.cost("m", 1_000_000, 0, cached) == pytest.approx(10.0)
 
+    def test_negative_cached_is_billed_in_full(self):
+        """负数命中数不得抬高成本: cost() 是公共方法,外部输入须校验后使用(P5)。"""
+        assert self._CACHED.cost("m", 1_000_000, 0, -500_000) == pytest.approx(10.0)
+
     def test_cached_over_prompt_is_clamped_and_never_negative(self):
         """网关口径异常时按输入总数夹取: 全部按缓存价,不得算出负成本。"""
         clamped = self._CACHED.cost("m", 1_000_000, 0, 5_000_000)