Preload variants when checking order promotionability to avoid an N+1

[ikraamg]

Evaluating a promotion's eligibility checks whether the order contains a non-promotionable product, reading each line item's variant and product with a separate query per line item. This runs every time a promotion is evaluated, including on order recalculation when promotions are present.

• preloads each line item's variant and product before the promotionable check
• variant and product loads stay constant regardless of order size
• adds a spec asserting line item variants load in a single query

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.69%. Comparing base (8d781ac) to head (1003344).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6499   +/-   ##
=======================================
  Coverage   89.68%   89.69%           
=======================================
  Files         993      993           
  Lines       20863    20864    +1     
=======================================
+ Hits        18712    18713    +1     
  Misses       2151     2151           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.