feat(categories): include parent_uuid in internal category responses

[roncodes]

Change

The internal Category resource now includes parent_uuid. Public responses don't.

Storefront is adding nested product and catalog categories, up to three levels. Its console already draws a category tree in the products sidebar (category-sidebar.js, childrenOf() by parent_uuid), but the resource never sent parent_uuid, so the tree always rendered flat. With the field, the console builds the tree from the flat list it already loads, without with_parent loading every parent. The console's category Ember model already has a parent_uuid attribute.

Tests

tests/Unit/Http/ResourceResponseContractsTest.php:

• internal payloads include parent_uuid;
• public payloads don't.

13 tests pass. php-cs-fixer: clean.

Release

Adds an "Added" entry to the v1.6.71 RELEASE.md. No migration: parent_uuid is an existing column.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (719b438) to head (c31a3f0).

Additional details and impacted files

@@                 Coverage Diff                 @@
##             release/v1.6.71      #316   +/-   ##
===================================================
  Coverage             100.00%   100.00%           
  Complexity              8931      8931           
===================================================
  Files                    473       473           
  Lines                  28056     28057    +1     
===================================================
+ Hits                   28056     28057    +1     

Flag	Coverage Δ
backend	100.00% <100.00%> (ø)

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