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_telemetry.py

@@ -318,7 +318,7 @@ CREATE TABLE llm_calls (
 _PRE_TENANT_INSERT = (
     "INSERT INTO llm_calls (call_id, model, provider, source_name, messages, response, "
     "prompt_tokens, completion_tokens, usage_source, latency_ms) "
-    "VALUES ('old-row', 'm', 'p', 's1', '[]', 'contract text', 1, 2, 'measured', 10)"
+    "VALUES ('old-row', 'm', 'p', 's1', '[]', 'old body', 1, 2, 'measured', 10)"
 )
 
 
@@ -399,12 +399,16 @@ class TestSQLiteCallerDimensionsAcceptance:
         conn.close()
         db.chmod(0o444)
 
-        recorder = SQLiteRecorder(db)  # 不得抛
-        assert recorder._conn is not None  # 补列失败 ≠ recorder 失能
-        await _record_minimal(recorder, call_id="doomed")  # 只读库写不进,但不得抛
-        recorder.close()
+        # finally 还原权限位: 任一断言先失败时,不还原会让 tmp_path 清理连带报错,
+        # 把"某条断言失败"的真因盖成一个无关的 PermissionError
+        try:
+            recorder = SQLiteRecorder(db)  # 不得抛
+            assert recorder._conn is not None  # 补列失败 ≠ recorder 失能
+            await _record_minimal(recorder, call_id="doomed")  # 只读库写不进,但不得抛
+            recorder.close()
+        finally:
+            db.chmod(0o644)
 
-        db.chmod(0o644)  # 还原,让 tmp_path 清理不受阻
         stale = sqlite3.connect(db)
         assert [r[1] for r in stale.execute("PRAGMA table_info(llm_calls)")] == (
             _EXPECTED_COLUMNS[:-2]
@@ -875,6 +879,25 @@ class TestEmitterCallerDimensions:
         )
         assert "研发" in rec.rows[0]["meta"]
 
+    async def test_non_finite_meta_value_drops_the_row_instead_of_poisoning_it(self):
+        """入口失守时 `allow_nan=False` 的真实结果: 整行降级丢弃,且不抛给调用方。
+
+        直接构造带 `nan` 的 `ChatRequest`(绕过 `validate_caller_dimensions` 这道
+        主防线,模拟将来某个新入口忘记校验)。没有 `allow_nan=False` 时,
+        `json.dumps` 会写出裸 `NaN` 字面量——PG 的 JSONB 会拒收,但 **SQLite 的
+        `meta` 是 TEXT 列不做校验**,那串非法 JSON 会被静默存进去,污染此后一切
+        按 JSON 解析 meta 的分析。宁可丢一行遥测,也不要一行毒数据。
+
+        同时断言不抛: 遥测的降级方向是"静默降级"(铁律),把调用方的一次正常
+        业务调用因为一个维度值炸掉,方向反了。
+        """
+        rec = _MemoryRecorder()
+        req = ChatRequest(messages=[{"role": "user", "content": "hi"}], meta={"k": float("nan")})
+        await TelemetryEmitter(rec).emit_terminal_failure(
+            request=req, call_id="c", latency_ms=1, error="dead"
+        )
+        assert rec.rows == []
+
 
 class TestCostWithCachedTier:
     """issue #3: 命中部分按缓存单价计费,避免 cost 系统性高估。"""