Fix Windows child process tests string escaping

The child process tests were failing on Windows because of incorrect
string escaping in the Python scripts passed to subprocesses.

The issues were:
1. Missing f-string prefix on parent script strings containing {repr(...)}
2. Incorrect escaping of newlines (\\\\n instead of \\n)
3. Incorrect quote escaping in nested scripts

These fixes ensure that:
- File paths are properly interpolated using f-strings
- Newlines are correctly escaped as \n in the child scripts
- Nested quotes use triple quotes instead of escaped quotes

Reported-by: fweinberger

Author: felixweinberger

tests/client/test_stdio.py

@@ -385,11 +385,11 @@ async def test_stdio_client_child_process_cleanup():
                 f.write('parent started\\n')
 
             # Child script that writes continuously
-            child_script = '''
+            child_script = f'''
             import time
             with open({repr(marker_file)}, 'a') as f:
                 while True:
-                    f.write(f"{{time.time()}}\\\\n")
+                    f.write(f"{{time.time()}}\\n")
                     f.flush()
                     time.sleep(0.1)
             '''
@@ -485,25 +485,25 @@ async def test_stdio_client_nested_process_tree():
             import os
 
             # Child will spawn grandchild and write to child file
-            child_script = '''import subprocess
+            child_script = f'''import subprocess
             import sys
             import time
 
             # Grandchild just writes to file
-            grandchild_script = \\"\\"\\"import time
+            grandchild_script = """import time
             with open({repr(grandchild_file)}, 'a') as f:
                 while True:
-                    f.write(f"gc {{time.time()}}\\\\\\\\n")
+                    f.write(f"gc {{time.time()}}\\n")
                     f.flush()
-                    time.sleep(0.1)\\"\\"\\"
+                    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:
                 while True:
-                    f.write(f"c {{time.time()}}\\\\\\\\n")
+                    f.write(f"c {{time.time()}}\\n")
                     f.flush()
                     time.sleep(0.1)'''
 
@@ -513,7 +513,7 @@ async def test_stdio_client_nested_process_tree():
             # Parent writes to its file
             with open({repr(parent_file)}, 'a') as f:
                 while True:
-                    f.write(f"p {{time.time()}}\\\\n")
+                    f.write(f"p {{time.time()}}\\n")
                     f.flush()
                     time.sleep(0.1)
             """
@@ -588,10 +588,10 @@ async def test_stdio_client_early_parent_exit():
             import signal
 
             # Child that continues running
-            child_script = '''import time
+            child_script = f'''import time
             with open({repr(marker_file)}, 'a') as f:
                 while True:
-                    f.write(f"child {{time.time()}}\\\\n")
+                    f.write(f"child {{time.time()}}\\n")
                     f.flush()
                     time.sleep(0.1)'''