test: close two always-green holes in the dimension tests

The cache-key test only asserted a hit, so a key degraded to a constant
would still pass it. Adding a namespace control group that must miss
proves the key still distinguishes inputs; verified by degrading
build_cache_key to a constant and watching the case go red.

The allow_nan=False branch had no test at all. A ChatRequest built with
a nan meta value (bypassing the entry validation, i.e. a future entry
point that forgets to validate) must drop the row and not raise;
verified red by removing allow_nan=False.

Also restore the read-only file permissions in a finally block, so a
failing assertion does not get masked by a PermissionError from tmp_path
cleanup; rename the warnings fixture to captured_warnings so it stops
shadowing the stdlib module; and drop a downstream business term from a
fixture value (zero-business-assumption rule).

tests/unit/test_client.py

@@ -232,13 +232,23 @@ class TestCallerDimensions:
         assert recorder.rows == []
 
     async def test_meta_does_not_enter_cache_key(self):
-        """仅 meta 不同必须仍命中缓存(F1): 进 key 会让存量缓存全量冷启动且不报错。"""
-        client = _client(cache=InMemoryCache(), cache_namespace="proj", cache_ttl_s=3600)
+        """仅 meta 不同必须仍命中缓存(F1): 进 key 会让存量缓存全量冷启动且不报错。
+
+        带对照组: 只断言"命中"的话,缓存 key 退化成常量(忽略一切输入)时本用例
+        照样绿——那是恒真断言。故再改一个**确实进 key** 的维度(namespace)断言
+        miss,证明 key 仍在区分输入,"meta 不进 key"才是被测出来的结论。
+        """
+        cache = InMemoryCache()  # 两个 client 共用一份存储,否则对照组的 miss 是白来的
+        client = _client(cache=cache, cache_namespace="proj", cache_ttl_s=3600)
         async with client:
             first = await client.chat(self._MSG, meta={"batch": "b-1"})
             second = await client.chat(self._MSG, meta={"batch": "b-2"})
         assert first.cache_hit is False and second.cache_hit is True
 
+        other_ns = _client(cache=cache, cache_namespace="other", cache_ttl_s=3600)
+        async with other_ns:
+            assert (await other_ns.chat(self._MSG, meta={"batch": "b-1"})).cache_hit is False
+
 
 class TestModelFingerprint:
     """配置级采样参数须进缓存身份,否则改 temperature 后仍读旧缓存(决策 C)。"""