fix: avoid closing stdio server streams

[HenryLee789]

Summary

• wrap default stdin/stdout with duplicated file descriptors so stdio_server does not take ownership of the real process streams
• close only the duplicated wrappers on exit
• add a regression test that verifies sys.stdin/sys.stdout remain open after stdio_server exits

Fixes #1933

Testing

• uv run pytest tests/server/test_stdio.py -q
• uv run ruff check src/mcp/server/stdio.py tests/server/test_stdio.py