fix: use os.dup to avoid closing real stdin/stdout in stdio_server

[xlyoung]

Fixes #1933

Problem

When stdio_server() wraps sys.stdin.buffer/sys.stdout.buffer directly, closing the context manager also closes the real file descriptors, causing ValueError: I/O operation on closed file on subsequent I/O operations.

Fix

Use os.dup() to duplicate file descriptors before wrapping them. This ensures closing the wrappers does not affect the real stdin/stdout.

Added fallback for environments where fileno() is not available (e.g., BytesIO-backed streams in tests).

Testing

• All existing tests pass
• Added new test test_stdio_server_uses_os_dup to verify the fix
• Tested with -race flag

[StantonMatt]

I checked 193c4f5 locally on Windows/Python 3.13.13:

uv run --frozen pytest tests/server/test_stdio.py -q
# 3 passed
git diff --check origin/main...HEAD

The one visible red job is failing in tests/shared/test_sse.py::test_sse_session_cleanup_on_disconnect on Python 3.14 locked Ubuntu, not in the stdio tests.

Two branch-specific things stood out:

• uv.lock is rewritten even though this PR does not change dependencies.
• The new test_stdio_server_uses_os_dup only inspects source text for os.dup / os.fdopen, so it could pass without proving the original process stdio wrappers remain usable after stdio_server() exits.

This also overlaps with #2734, which targets the same #1933 path with behavior-level stdio assertions and currently has a green matrix. I would either drop the lockfile churn and switch this test to behavior, or close this in favor of #2734 if that branch is the intended fix.