Fix stdio_server from closing process stdio handles

[aframedelta]

Summary

• prevent stdio_server() from closing underlying process stdio handles
• add a regression test that verifies stdio remains usable after server shutdown

Why

Closes #1933.

stdio_server() wraps sys.stdin.buffer / sys.stdout.buffer in a TextIOWrapper. When the async wrapper is closed, the underlying process stdio can be closed too, causing subsequent print() calls to fail with ValueError: I/O operation on closed file.

Changes

• introduce _NonClosingTextIOWrapper in mcp.server.stdio
• use it for default process stdio wrapping
• add test_stdio_server_does_not_close_process_stdio regression test

Validation

• . .venv/bin/activate && pytest tests/server/test_stdio.py -q
  • 2 passed