Upgrade rack dependency to fix potential CVEs:

• CVE-2025-49007 Fix ReDoS in multipart request.

• CVE-2025-61772 Multipart parser buffers unbounded per-part headers, enabling DoS (memory exhaustion)

• CVE-2025-61771 Multipart parser buffers large non‑file fields entirely in memory, enabling DoS (memory exhaustion)

• CVE-2025-61770 Unbounded multipart preamble buffering enables DoS (memory exhaustion)

• CVE-2025-61780 Improper handling of headers in Rack::Sendfile may allow proxy bypass.

• CVE-2025-61919 Unbounded read in Rack::Request form parsing can lead to memory exhaustion.

See release notes: https://github.com/rack/rack/blob/main/CHANGELOG.md#3119---2025-11-03