test: golden the key and fingerprint literals for a declared tier

The absent-tier side has had goldens since issue #4; the side that
actually carries a tier had none, so swapping str() for repr() in the
cache key changed the literal without turning the suite red. Pin both
literals. Note the fingerprint's json.dumps emits the same bytes for an
Effort member and its .value, so that pair is indistinguishable by any
test -- the golden pins the literal itself instead.

tests/unit/test_cache.py

@@ -155,6 +155,29 @@ class TestKeyFormula:
         ) == ("pgw:cache:c54544e8672f4c91373b4a72716a88497445b440b89445aa5379b356b228f58b")
 
 
+    def test_declared_tier_key_is_a_golden(self):
+        """配了档位那一侧同样要有 golden: 字面量变了就是所有该档缓存冷启动。
+
+        存量(不表态)那侧的 golden 由 `test_absent_tier_keeps_legacy_key` 守着,
+        而"档位怎么写进 key"此前没有任何字面量断言——变异实测把 `str(...)` 换成
+        `repr(...)`,全套件依然全绿(2026-09-05 独立验证查出)。
+        """
+        assert build_cache_key(
+            "qwen-max",
+            [{"role": "user", "content": "hi"}],
+            "proj",
+            None,
+            reasoning_effort=Effort.LOW,
+        ) == ("pgw:cache:21d7be93729635b27d4ee54e0e7e7310554bb04faf08919f8ce83794ac33f575")
+        assert build_cache_key(
+            "qwen-max",
+            [{"role": "user", "content": "hi"}],
+            "proj",
+            "s1",
+            reasoning_effort=Effort.NONE,
+        ) == ("pgw:cache:44f4f1ce1ee39e5003a27f4f21a531554cb54d1c66e936d11008d4ff06ddc5e6")
+
+
 class _Terminal:
     def __init__(self, response):
         self.response = response