All error paths in the ReplaceAllFunc callback returned nil, which
ReplaceAllFunc treats as an empty replacement, silently erasing the
guard (if err != nil) fired, every subsequent include in the file
was also erased. Return spec (the original directive bytes) instead
so failed includes are preserved and subsequent ones are not lost.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>