Revert "fix(tests): stabilize child process cleanup under CI load"

This reverts commit 8f1b17f.

Author: nypdmax

tests/client/test_stdio.py

@@ -480,17 +480,12 @@ def handle_term(sig, frame):
             # Let child start writing
             await anyio.sleep(0.5)
 
-            # Verify child is writing. This can be slow/flaky under CI load, so poll up to a bounded timeout.
+            # Verify child is writing
             if os.path.exists(marker_file):  # pragma: no branch
-                initial_size = os.path.getsize(marker_file)
-                deadline = anyio.current_time() + 5.0
-                while anyio.current_time() < deadline:
-                    await anyio.sleep(0.1)
-                    current_size = os.path.getsize(marker_file)
-                    if current_size > initial_size:  # pragma: no branch
-                        break
-                else:  # pragma: no cover
-                    raise AssertionError("Child should be writing")
+                size1 = os.path.getsize(marker_file)
+                await anyio.sleep(0.3)
+                size2 = os.path.getsize(marker_file)
+                assert size2 > size1, "Child should be writing"
 
             # Terminate - this will kill the process group even if parent exits first
             await _terminate_process_tree(proc)