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/integration/test_postgres_telemetry.py

@@ -424,7 +424,7 @@ CREATE TABLE {schema}.llm_calls (
 _PRE_TENANT_INSERT = (
     "INSERT INTO {schema}.llm_calls (call_id, model, provider, source_name, messages, response, "
     "prompt_tokens, completion_tokens, usage_source, latency_ms) "
-    "VALUES ($1, 'm', 'p', 's1', '[]', 'contract text', 1, 2, 'measured', 10)"
+    "VALUES ($1, 'm', 'p', 's1', '[]', 'old body', 1, 2, 'measured', 10)"
 )
 
 
@@ -434,8 +434,12 @@ def _search_path_dsn(dsn: str, schema: str) -> str:
 
 
 @pytest.fixture
-async def warnings():
-    """捕获库发出的 WARNING;loguru 不经标准 logging,pytest 的 caplog 抓不到。"""
+async def captured_warnings():
+    """捕获库发出的 WARNING;loguru 不经标准 logging,pytest 的 caplog 抓不到。
+
+    名字避开裸 `warnings`: 那会遮蔽标准库模块名,本文件将来任何一次
+    `import warnings` 都会与它静默互相顶掉,而报错点离真因很远。
+    """
     from loguru import logger
 
     messages: list[str] = []
@@ -608,7 +612,7 @@ class TestCallerDimensionsAcceptance:
             await conn.close()
 
     async def test_backfill_failure_degrades_per_row_not_wholesale(
-        self, least_privilege_pre_tenant_dsn, warnings
+        self, least_privilege_pre_tenant_dsn, captured_warnings
     ):
         """补列失败的降级方向: 记 warning、不置 `_failed`、后续 INSERT 仍照发。
 
@@ -619,8 +623,8 @@ class TestCallerDimensionsAcceptance:
         try:
             await _record_minimal(recorder, call_id=_cid("lpp1"))  # 不得抛
             assert recorder._failed is False
-            assert any("补列失败" in m for m in warnings)
+            assert any("补列失败" in m for m in captured_warnings)
             # 缺列的表上 INSERT 必然失败;逐行 warning 正是"INSERT 照发了"的证据
-            assert any("写入失败" in m for m in warnings)
+            assert any("写入失败" in m for m in captured_warnings)
         finally:
             await recorder.aclose()