bug: open(<&, $fh) dup fails on mocked filehandles

[toddr-bot]

open(my $dup, "<&", $fh) (and >&, <&=, >&=) fails with "Bad file descriptor" when $fh is a mocked filehandle.

Reproduce

use Test::MockFile ();
my $mock = Test::MockFile->file("/tmp/test", "hello\n");
open(my $fh, "<", "/tmp/test") or die;
open(my $dup, "<&", $fh) or die "dup: $!";  # Dies: Bad file descriptor

Cause

The <& mode is not in __open's supported mode list (> < >> +< +> +>>), so it falls through to CORE::open. Since mocked handles have synthetic filenos (FILENO dies), the kernel-level dup fails.

Fix

PR #424 detects dup modes early in __open and creates a new tied handle sharing the same mock data when the source handle is mocked.