Fix Windows process termination in tests

The WindowsProcessWrapper was missing a 'popen' attribute that
_terminate_process_with_children looks for when extracting the PID
on Windows. This caused the termination to fail silently.

Added the popen attribute to WindowsProcessWrapper to ensure proper
PID extraction and process termination with taskkill /T.

Reported-by: fweinberger

Author: felixweinberger

tests/client/test_stdio.py

@@ -34,6 +34,8 @@ class WindowsProcessWrapper:
     def __init__(self, popen):
         self.pid = popen.pid
         self._popen = popen
+        # Add popen attribute for compatibility with _terminate_process_with_children
+        self.popen = popen
 
     async def wait(self):
         while self._popen.poll() is None: