Allow absolute paths for dynamic attachments

In EmailAttachmentEvaluator::evaluateAttachments(), the dynamic-attachments branch unconditionally strips a leading slash before calling realpath(). The in-source comment explains the intent: a friendly normalization for admins who type /some/relative/inside-docroot, thinking it means relative to the project root.

But the strip also fires on legitimate absolute paths. /var/www/example.com/web/sites/default/files/foo.pdf is rewritten to var/www/example.com/web/sites/default/files/foo.pdf, then realpath() tries to resolve it relative to PHP's CWD, fails, and the file is silently reported as attachment not found even though it's readable as the original absolute path. Any code that populates attachment_path with a real filesystem path could be affected by this.