SolidusAdmin products/stock component fixes

[elia]

Summary

• add tests for .from_products
• add .from_variant to support stock information for a single variant

Checklist

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The following are mandatory for all PRs:

• I have written a thorough PR description.
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• 📖 I have updated the README to account for my changes.
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[codecov]

Codecov Report

Merging #5443 (cbcbb90) into main (e097f4a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #5443   +/-   ##
=======================================
  Coverage   88.89%   88.89%           
=======================================
  Files         612      612           
  Lines       14798    14800    +2     
=======================================
+ Hits        13154    13156    +2     
  Misses       1644     1644           

Files	Coverage Δ
...mponents/solidus_admin/products/stock/component.rb	100.00% <100.00%> (ø)

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