test: pin the cooldown assertion to a fake clock

The status snapshot reports elapsed time, so asserting retry_after_s
against the real monotonic clock was really asserting that a few lines
of code take zero time; it failed at 59.99993 vs 60.0. The recorder
already accepts an injected clock for exactly this reason.

tests/unit/test_telemetry.py

@@ -1930,7 +1930,9 @@ class TestSQLiteStatusVisibility:
 class TestPostgresStatusVisibility:
     """PG 侧的降级必须能被下游查到(计划 T2 建立可见性,T5 改判据)。"""
 
-    def _recorder(self, conn):
+    def _recorder(self, conn, now=None):
+        """假时钟是缺省: 快照里的 `retry_after_s`/`degraded_for_s` 是**时间差**,
+        用真实时钟断言就等于断言"这几行代码零耗时",是设计上就会间歇红的用例。"""
         from polygateway.telemetry.postgres import PostgresRecorder
 
         return PostgresRecorder(
@@ -1939,6 +1941,7 @@ class TestPostgresStatusVisibility:
             auto_migrate=True,
             pool_max=_TEST_POOL_MAX,
             write_timeout_s=_TEST_WRITE_TIMEOUT_S,
+            now=now or _FakeClock(),
         )
 
     async def test_unusable_table_shows_up_in_the_status(self, captured_warnings):
@@ -1950,7 +1953,7 @@ class TestPostgresStatusVisibility:
         status = recorder.telemetry_status
         assert status.degraded is True and status.fatal is False  # 环境级: 建了表就该自愈
         assert status.dropped_rows == 1  # 降级那一次调用本身也丢了一行
-        assert status.retry_after_s == pytest.approx(_DEGRADE_COOLDOWN_S)
+        assert status.retry_after_s == _DEGRADE_COOLDOWN_S  # 假时钟不动,余额恰是整个冷却期
 
     async def test_healthy_recorder_is_not_degraded(self):
         recorder = self._recorder(_FakePgConn(list(_EXPECTED_COLUMNS)))