fix: prevent filehandle reference leak in stat override

[Koan-Bot]

Summary

• Fix $_last_call_for in _check() to only cache string filenames, not filehandle references
• Prevents filehandle garbage collection being blocked by a retained reference
• Adds regression test for the leak

Root Cause

_check() stored its $file argument in $_last_call_for for -X _ caching support. When the argument was a filehandle reference (e.g. from -S $fh), this prevented the filehandle from being garbage collected, keeping the underlying file descriptor open.

This caused "spooky action-at-a-distance" bugs: a socketpair read could hang because a dup'd write-end filehandle was kept alive by the leaked reference.

Fix

# Before:
$_last_call_for = $file;

# After:
$_last_call_for = ref($file) ? undef : $file;

Only string filenames are cached. Filehandle references pass through without being retained.

Test plan

• New test t/fh-ref-leak.t verifies filehandles are GC'd after file checks
• Existing -X _ tests still pass (string filename caching preserved)

Ref: cpanel/Test-MockFile#179

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