Fix test_stdio_client_immediate_completion race condition

Use Python script instead of echo command to avoid BrokenResourceError
on fast Linux systems where echo exits before streams are fully set up.

Author: felixweinberger

tests/client/test_stdio.py

@@ -150,17 +150,14 @@ async def test_stdio_client_immediate_completion():
     when processes exit normally and quickly.
     """
 
-    # Use a command that exits immediately
-    if sys.platform == "win32":
-        server_params = StdioServerParameters(
-            command="cmd",
-            args=["/c", "echo", "hello"],  # Windows: echo and exit
-        )
-    else:
-        server_params = StdioServerParameters(
-            command="echo",
-            args=["hello"],  # Unix: echo and exit
-        )
+    # Use a Python script that prints and exits immediately
+    # This is more consistent across platforms than echo
+    script_content = "print('hello')"
+    
+    server_params = StdioServerParameters(
+        command=sys.executable,
+        args=["-c", script_content],
+    )
 
     start_time = time.time()