Support array response headers in CGI

[OskarEichler]

Rack 3 permits response header values to be either a String or an Array of Strings. The CGI handler calls split directly on every value, so it raises NoMethodError for a conforming array value such as multiple set-cookie headers.

Wrap each value with Array(...), emit every item separately, and retain the existing newline splitting for legacy String values.

Verification:

• deterministic CGI model fails on 2.3.1/current and passes this branch
• full suite: 76 runs / 194 assertions / 0 failures
• Ruby syntax, warning-mode load, package build and diff checks pass on Ruby 4.0.6
• combined candidate also passes 74 runs / 182 assertions on Ruby 3.2.11

No breaking API change.

[ioquatix]

It may be good to include a test that would otherwise fail.

[OskarEichler]

Added a CGI regression covering an array-valued set-cookie header while retaining the existing newline-splitting behavior for String values. The focused test and full local suite pass: 75 runs, 179 assertions, 0 failures. I also checked the failing external CI job: the rackup tests pass there; it fails later because current Falcon now requires Ruby 3.3 while that external job runs Ruby 3.2, so it is unrelated to this change.