Shipping category foreign keys

[mamhoff]

Summary

Adds foreign key constraints and validations to tables related to shipping categories, specifically spree_products and spree_shipping_method_categories.

Extracted from #6240

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.95%. Comparing base (1061e56) to head (4b87bdd).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6278      +/-   ##
==========================================
- Coverage   88.95%   88.95%   -0.01%     
==========================================
  Files         860      860              
  Lines       18423    18422       -1     
==========================================
- Hits        16388    16387       -1     
  Misses       2035     2035              

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