[ruby/zlib] Check that Zlib::OS_CODE is OS_WIN32 on Windows

test_os_code only compared the written os_code with Zlib::OS_CODE itself, so it could not catch OS_CODE falling back to OS_UNIX.

https://github.com/ruby/zlib/commit/104cbd525f

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/zlib/test_zlib.rb

@@ -677,6 +677,7 @@ if defined? Zlib
           assert_equal(Zlib::OS_CODE, f.os_code)
         end
       }
+      assert_equal(Zlib::OS_WIN32, Zlib::OS_CODE) if /mswin|mingw/ =~ RUBY_PLATFORM
     end
 
     def test_orig_name