[3.12] gh-106186: Don't report MultipartInvariantViolationDefect for valid multipart emails when parsing header only (GH-107016) (#107111)

(cherry picked from commit c65592c4d6d7552fb6284442906a96a6874cb266)

Co-authored-by: htsedebenham <31847376+htsedebenham@users.noreply.github.com>