fix: dup file descriptors in stdio_server to avoid closing real stdin/stdout

[giulio-leone]

Problem

Running a server with transport="stdio" closes the real sys.stdin / sys.stdout when the server exits. Any subsequent stdio operation raises:

ValueError: I/O operation on closed file.

Root Cause

TextIOWrapper(sys.stdin.buffer) shares the underlying file descriptor with sys.stdin. When the TextIOWrapper is closed (or garbage-collected) after the server exits, it also closes sys.stdin.buffer, making any subsequent stdio operation fail.

Solution

Use os.dup() to duplicate the file descriptor before wrapping it in TextIOWrapper. Closing the wrapper then only closes the duplicate fd while leaving the original process handles intact.

# Before (broken)
stdin = anyio.wrap_file(TextIOWrapper(sys.stdin.buffer, encoding="utf-8"))

# After (fixed)
stdin = anyio.wrap_file(
    TextIOWrapper(os.fdopen(os.dup(sys.stdin.fileno()), "rb"), encoding="utf-8")
)

Testing

• All 15 stdio-related tests pass across 2 consecutive runs
• Verified manually that sys.stdin.fileno() remains valid after closing the duplicated wrapper

Fixes #1933