Fix Windows path handling in child process tests

Windows file paths contain backslashes that were being interpreted as
escape sequences when embedded in Python code strings, causing syntax
errors like "truncated \UXXXXXXXX escape".

The fix:
1. Added escape_path_for_python() helper that converts backslashes to
   forward slashes (which work on all platforms)
2. Updated all file path interpolations to use this helper
3. Fixed indentation in the grandchild script

This ensures Windows paths like C:\Users\... are properly handled
when embedded in dynamically generated Python scripts.

Reported-by: fweinberger

Author: felixweinberger

tests/client/test_stdio.py

@@ -22,6 +22,12 @@
 python: str = shutil.which("python")  # type: ignore
 
 
+def escape_path_for_python(path: str) -> str:
+    """Escape a file path for use in Python code strings."""
+    # Use forward slashes which work on all platforms and don't need escaping
+    return repr(path.replace('\\', '/'))
+
+
 class WindowsProcessWrapper:
     """Minimal wrapper for subprocess.Popen to work with anyio-style process interface."""
 
@@ -381,13 +387,13 @@ async def test_stdio_client_child_process_cleanup():
             import os
 
             # Mark that parent started
-            with open({repr(parent_marker)}, 'w') as f:
+            with open({escape_path_for_python(parent_marker)}, 'w') as f:
                 f.write('parent started\\n')
 
             # Child script that writes continuously
             child_script = f'''
             import time
-            with open({repr(marker_file)}, 'a') as f:
+            with open({escape_path_for_python(marker_file)}, 'a') as f:
                 while True:
                     f.write(f"{time.time()}")
                     f.flush()
@@ -491,17 +497,17 @@ async def test_stdio_client_nested_process_tree():
 
             # Grandchild just writes to file
             grandchild_script = \"\"\"import time
-with open({repr(grandchild_file)}, 'a') as f:
-    while True:
-        f.write(f"gc {{time.time()}}")
-        f.flush()
-        time.sleep(0.1)\"\"\"
+            with open({escape_path_for_python(grandchild_file)}, 'a') as f:
+                while True:
+                    f.write(f"gc {{time.time()}}")
+                    f.flush()
+                    time.sleep(0.1)\"\"\"
 
             # Spawn grandchild
             subprocess.Popen([sys.executable, '-c', grandchild_script])
 
             # Child writes to its file
-            with open({repr(child_file)}, 'a') as f:
+            with open({escape_path_for_python(child_file)}, 'a') as f:
                 while True:
                     f.write(f"c {time.time()}")
                     f.flush()
@@ -511,7 +517,7 @@ async def test_stdio_client_nested_process_tree():
             subprocess.Popen([sys.executable, '-c', child_script])
 
             # Parent writes to its file
-            with open({repr(parent_file)}, 'a') as f:
+            with open({escape_path_for_python(parent_file)}, 'a') as f:
                 while True:
                     f.write(f"p {time.time()}")
                     f.flush()
@@ -589,7 +595,7 @@ async def test_stdio_client_early_parent_exit():
 
             # Child that continues running
             child_script = f'''import time
-            with open({repr(marker_file)}, 'a') as f:
+            with open({escape_path_for_python(marker_file)}, 'a') as f:
                 while True:
                     f.write(f"child {time.time()}")
                     f.flush()