fix(stdio): flush sys.stdout before restoring fd 1 in stdio_server (#3821)

Author: thinhnx128

src/mcp/server/stdio.py

@@ -212,6 +212,8 @@ async def stdout_writer():
             yield read_stream, write_stream
     finally:
         if restore_stdout is not None:
+            with suppress(OSError, ValueError):
+                sys.stdout.flush()
             restore_stdout()
         if restore_stdin is not None:
             restore_stdin()