fix: add missing returned attribute for some summary models

[duffn]

No description provided.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (ab12e30) 100.00% compared to head (7970a2b) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main        #5   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           28        28           
  Lines          396       400    +4     
=========================================
+ Hits           396       400    +4     

Impacted Files	Coverage Δ
lib/harkness/models/character_list.rb	100.00% <ø> (ø)
lib/harkness/models/creator_list.rb	100.00% <ø> (ø)
lib/harkness/models/comic_list.rb	100.00% <100.00%> (ø)
lib/harkness/models/event_list.rb	100.00% <100.00%> (ø)
lib/harkness/models/series_list.rb	100.00% <100.00%> (ø)
lib/harkness/models/story_list.rb	100.00% <100.00%> (ø)

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