fix: preserve stdio streams in server transport

[pranjalbhatia710]

Summary

• duplicate the process stdin/stdout file descriptors before wrapping them for the stdio server transport
• close only the duplicated wrapper streams on shutdown, preserving the original process-level streams
• extend the stdio regression test to assert the original sys.stdin/sys.stdout wrappers remain usable after the server exits

Fixes #1933

Test plan

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