Email header injection via malformed comment folding

Header injection in CPython's email module allows remote authenticated users to add fields to generated messages through crafted addresses. When a modern email policy flattens an address comment made only of a long sequence of non-foldable characters, _header_value_parser.py forces a line wrap without the required leading whitespace, so the remaining comment is parsed as a new header field. Reachability requires an application to accept an unsanitized user-controlled address and flatten the message; the bounded consequence is unauthorized modification of generated email headers.