fix: preserve stdin/stdout file descriptors after server exits

[haroldfabla2-hue]

Description

Using os.dup() to create copies of file descriptors prevents closing the original sys.stdin/sys.stdout when the TextIOWrapper is closed.

Fixes

Fixes #1933

Changes

• Use os.dup() to create duplicate file descriptors for stdin/stdout
• Use os.fdopen() with closefd=True to manage the duplicated fd
• This prevents ValueError: I/O operation on closed file after the server exits

Testing

The fix can be tested by running:

from mcp.server.fastmcp import FastMCP

mcp = FastMCP("Demo")
mcp.run(transport="stdio")
print("?")

After exiting the server with Ctrl+D, the print should work without errors.