OrderPromotions: Validate promotion uniqueness per order

[mamhoff]

Summary

OrderPromotion objects connect a promotion to an order (e.g. through entering a promotion code). If an order is already connected to an order through an order promotion, we don't need to add a second record.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.50%. Comparing base (3896980) to head (25ab4c5).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6424   +/-   ##
=======================================
  Coverage   89.50%   89.50%           
=======================================
  Files         981      981           
  Lines       20476    20477    +1     
=======================================
+ Hits        18327    18328    +1     
  Misses       2149     2149           

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