Fix failing API promotions specs

[MadelineCollier]

Summary

Three of these examples were failing with the following error:

 NameError:
   uninitialized constant Spree::Api::OpenStruct

         OpenStruct.new(
         ^^^^^^^^^^

Requiring ostruct and ensuring the correct scope is used when referencing OpenStruct fixes the errors.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.23%. Comparing base (72920b8) to head (f7cea4b).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5859      +/-   ##
==========================================
+ Coverage   87.87%   89.23%   +1.35%     
==========================================
  Files         477      752     +275     
  Lines       11656    17510    +5854     
==========================================
+ Hits        10243    15625    +5382     
- Misses       1413     1885     +472     

Flag	Coverage Δ
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[tvdeyen]

Thanks!

[mamhoff]

Thank you, much appreciated!