Avoid an N+1 loading stock on the admin products index

[ikraamg]

The solidus_admin products index renders each product's stock via total_on_hand, which loaded stock_items for every variant because variants_including_master and their stock items weren't eager-loaded. The page's query count grew with the number of products and variants.

• eager-loads variants_including_master with their stock items and stock locations
• the index issues a single stock_items query regardless of page size
• adds a request spec asserting the index renders and has no stock N+1

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.68%. Comparing base (8d781ac) to head (10f869c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6495   +/-   ##
=======================================
  Coverage   89.68%   89.68%           
=======================================
  Files         993      993           
  Lines       20863    20863           
=======================================
  Hits        18712    18712           
  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.