[14.0][FIX] website_sale_extra_fields: slug_name NOT NULL on fresh install

[eantones]

Summary

• Add pre_init_hook to pre-create the slug_name column on product_public_category and fill existing NULL rows with a placeholder before _auto_init runs
• Prevents ERROR: column "slug_name" contains null values when installing on a database that already has product.public.category records (e.g. from website_sale demo data)
• Standard OCA pattern for required stored computed fields on inherited models

Test plan

• Install website_sale_extra_fields on a fresh database with demo data enabled — should not crash
• Verify slug_name is computed correctly after install

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.58%. Comparing base (ce814c2) to head (6ad89cf).

Additional details and impacted files

@@            Coverage Diff             @@
##             14.0     #870      +/-   ##
==========================================
+ Coverage   50.53%   50.58%   +0.05%     
==========================================
  Files         979      980       +1     
  Lines       16685    16692       +7     
  Branches     3558     3558              
==========================================
+ Hits         8432     8444      +12     
+ Misses       8034     8033       -1     
+ Partials      219      215       -4     

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