Preload variant and product when calculating taxes to avoid an N+1

[ikraamg]

Calculating order taxes resolves each line item's tax category through its variant, falling back to the product when the variant has none. With those associations unloaded, that was two queries per line item, and taxes are recalculated on every order recalculation.

• preloads each line item's variant and product before calculating taxes
• 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 92.02%. Comparing base (b6fbbe7) to head (57af561).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6497   +/-   ##
=======================================
  Coverage   92.02%   92.02%           
=======================================
  Files        1035     1035           
  Lines       21125    21126    +1     
=======================================
+ Hits        19440    19441    +1     
  Misses       1685     1685           

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