Flush the stdio subprocess's coverage data before the clean-exit line (#2840)

tests/interaction/transports/test_stdio.py

@@ -56,10 +56,12 @@ async def test_tool_call_and_notification_round_trip_over_a_stdio_subprocess(
     notification before the call returns; the server exits when the transport closes its
     stdin.
     """
-    # After stdin closes, the child must unwind, write the clean-exit line, and let coverage's
-    # atexit hook persist its subprocess data file before escalation. The production 2s default
-    # was too tight on slow Windows runners: the child was killed mid-atexit (test stayed green)
-    # and the silently missing data file tripped the 100% coverage gate. Not under test.
+    # After stdin closes, the child must unwind, flush its subprocess coverage data, and write
+    # the clean-exit line before escalation (the server saves coverage *before* printing, so a
+    # post-print kill can no longer silently lose the data file -- see _stdio_server.main). The
+    # production 2s default is too tight for the unwind+save tail on loaded Windows runners
+    # (measured in-situ p99 of the whole test is ~7s); a kill before the print fails the stderr
+    # assertion below loudly rather than tripping the coverage gate. Not under test.
     monkeypatch.setattr(stdio, "PROCESS_TERMINATION_TIMEOUT", 10.0)
 
     received: list[LoggingMessageNotificationParams] = []