Implement recursive process tree termination for Windows

The taskkill /T command doesn't reliably kill all child processes
when they're created by Python subprocesses. This implements a
recursive solution using WMI to:

1. Find all child processes using wmic
2. Recursively kill all children first (depth-first)
3. Finally kill the parent process

This ensures that entire process trees are properly terminated,
including nested processes like parent->child->grandchild.

Reported-by: fweinberger

Author: felixweinberger

src/mcp/client/stdio/__init__.py

@@ -252,6 +252,49 @@ async def _create_platform_compatible_process(
     return process
 
 
+async def _kill_process_tree_windows(pid: int) -> None:
+    """
+    Recursively kill a process and all its children on Windows.
+    Uses WMI to find child processes since taskkill /T doesn't always work.
+    """
+    # First, get all child processes
+    children_pids = set()
+    
+    # Use wmic to get child processes
+    result = await anyio.to_thread.run_sync(
+        subprocess.run,
+        ["wmic", "process", "where", f"ParentProcessId={pid}", "get", "ProcessId", "/FORMAT:VALUE"],
+        capture_output=True,
+        shell=False,
+        check=False,
+        text=True,
+    )
+    
+    if result.returncode == 0 and result.stdout:
+        # Parse child PIDs
+        for line in result.stdout.strip().split('\n'):
+            if line.startswith('ProcessId='):
+                try:
+                    child_pid = int(line.split('=')[1].strip())
+                    if child_pid > 0:
+                        children_pids.add(child_pid)
+                except (ValueError, IndexError):
+                    pass
+    
+    # Recursively kill all children first
+    for child_pid in children_pids:
+        await _kill_process_tree_windows(child_pid)
+    
+    # Finally kill the parent process
+    await anyio.to_thread.run_sync(
+        subprocess.run,
+        ["taskkill", "/F", "/PID", str(pid)],
+        capture_output=True,
+        shell=False,
+        check=False,
+    )
+
+
 async def _terminate_process_with_children(process: Process | FallbackProcess, timeout: float = 2.0) -> None:
     """
     Terminate a process and all its children across platforms.
@@ -306,14 +349,8 @@ async def _terminate_process_with_children(process: Process | FallbackProcess, t
             with anyio.fail_after(timeout):
                 await process.wait()
         except TimeoutError:
-            # Force kill using taskkill for tree termination
-            # The /T flag kills the process tree when processes share the same console
-            await anyio.to_thread.run_sync(
-                subprocess.run,
-                ["taskkill", "/F", "/T", "/PID", str(pid)],
-                capture_output=True,
-                shell=False,
-                check=False,
-            )
+            # On Windows, we need to recursively kill all child processes
+            # because taskkill /T doesn't always work with Python subprocesses
+            await _kill_process_tree_windows(pid)
         except ProcessLookupError:
             pass