CVE-2026-26962: Rack's improper unfolding of folded multipart headers preserves CRLF in parsed parameter values

Rack::Multipart::Parser unfolds folded multipart part headers incorrectly. When a multipart header contains an obs-fold sequence, Rack preserves the embedded CRLF in parsed parameter values such as filename or name instead of removing the folded line break during unfolding.

As a result, applications that later reuse those parsed values in HTTP response headers may be vulnerable to downstream header injection or response splitting.