Eager load line item details on the admin order page to avoid N+1s

[ikraamg]

The admin order page loaded each line item's variant, product, images and option values lazily while rendering the cart, so the page's query count grew with the number of line items on the order.

• eager loads each line item's variant, product, variant images and option values
• the page's query count stays constant regardless of order size
• adds a request spec asserting the page renders and loads line item variants 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 (cdff698).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6500   +/-   ##
=======================================
  Coverage   92.02%   92.02%           
=======================================
  Files        1035     1035           
  Lines       21125    21125           
=======================================
  Hits        19440    19440           
  Misses       1685     1685           

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