test: document supervisor log timeout helper

Author: Elioooon

tests/src/unit/test_tools_utility_supervisor_logs.py

@@ -445,6 +445,7 @@ async def test_supervisor_log_fetch_has_overall_deadline(
         mock_client.timeout = 0.01
 
         async def _hang_forever(*_args, **_kwargs):
+            """Model a Supervisor response that never completes."""
             await asyncio.Event().wait()
 
         inner_client.get.side_effect = _hang_forever