PhpWeasyPrint: shell command injection via configurable WeasyPrint binary path due to inverted is_executable() guard (mirror of KnpLabs/snappy GHSA-vpr4-p6fq-85jc)

PhpWeasyPrint is a PHP library allowing PDF generation from a URL or an HTML page. Prior to version 2.5.1, `pontedilana/php-weasyprint` builds the shell command for WeasyPrint by passing the binary path through `escapeshellarg()` first and then checking the *quoted* result with `is_executable()`. On POSIX `escapeshellarg('/usr/local/bin/weasyprint')` returns `'/usr/local/bin/weasyprint'` with the single-quote characters as part of the string, so `is_executable()` looks for a file whose actual name includes those quotes. That file never exists, the "safe" branch is dead code, and the raw `$binary` string (set via the constructor or `setBinary()`) flows directly into `Symfony\Component\Process\Process::fromShellCommandline()`. Any deployment whose binary path is sourced from configuration, an environment variable, or a per-tenant setting reaches a shell-command-injection sink. The library is documented as a one-to-one substitute for KnpLabs/snappy and inherited the exact pre-fix codepath KnpLabs patched in GHSA-vpr4-p6fq-85jc. PhpWeasyPrint version 2.5.1 contains a patch for the issue.