tests: mark unreachable zero-iteration branch in POLLHUP monitor loop

The while loop at line 144 always executes at least once — POLLHUP fires
after the write end is closed, not before. The zero-iteration branch
(condition False on first check) is structurally unreachable in this test
but triggers a coverage miss under branch coverage.

Mark with pragma: no branch, consistent with the pattern used elsewhere
in the test suite (e.g. test_notification_response.py).

Author: Bortlesboat

tests/server/test_stdio.py

@@ -141,7 +141,7 @@ async def test_stdin_eof_monitor_ignores_pollin_events():
 
                 # Wait for the monitor to detect POLLHUP and cancel.
                 with anyio.fail_after(5):
-                    while not tg.cancel_scope.cancel_called:
+                    while not tg.cancel_scope.cancel_called:  # pragma: no branch
                         await anyio.sleep(0.05)
     finally:
         os.close(read_fd)