validateInlineJs unescaped the shell body with four chained replaces,
so a backslash pair followed by n or t was unescaped twice: the pair
became one backslash, then the new backslash-n became a newline. A
valid hook command such as console.log('\\n') failed validation with
an unterminated string. CodeQL flags the chain as double unescaping.
One regex pass maps each escape exactly once. The regression test
feeds the escaped-backslash program through the validator CLI.
