fix: prevent stdio_server from closing process stdio handles

[adityuhkapoor]

Motivation and Context

Fixes #1933. stdio_server() wraps sys.stdin.buffer and sys.stdout.buffer in TextIOWrapper. When the wrapper gets garbage-collected, it closes the underlying buffer, killing process stdio. Any subsequent print() or stdin usage raises ValueError: I/O operation on closed file.

os.dup() gives the wrapper its own copy of the file descriptor, so closing it doesn't affect the original process stdio.

How Has This Been Tested?

• Regression test that monkeypatches sys.stdin/sys.stdout with real pipe-backed file descriptors, runs stdio_server(), then asserts the original fds are still open via os.fstat().
• Verified the test fails without the fix and passes with it.
• Full test suite: 1139 passed, 0 failures.

Breaking Changes

No.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed