gh-143935: Email preserve parens when folding comments (#143936)

Fix a bug in the folding of comments when flattening an email message
using a modern email policy. Comments consisting of a very long sequence of
non-foldable characters could trigger a forced line wrap that omitted the
required leading space on the continuation line, causing the remainder of
the comment to be interpreted as a new header field. This enabled header
injection with carefully crafted inputs.

Co-authored-by: Denis Ledoux <dle@odoo.com>