Fix stdio_server closing process stdio

[GitAashishG]

Summary

• duplicate stdin/stdout file descriptors before wrapping them in stdio_server
• close only the duplicated async wrappers during teardown so process stdio stays usable
• add a regression test covering post-shutdown writes to sys.stdout

Testing

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

Closes #1933