feat(products): expose add-on category limits on public product payloads

[roncodes]

Summary

The public product payload (GET storefront/v1/products/{id} and every resource that embeds products) now includes each add-on category's limits, so apps can enforce them before checkout:

• addon_categories[].is_required (boolean, false when unset)
• addon_categories[].max_selectable (integer, or null for no limit)

The internal (Console) payload gains the same two fields. Both already exist on ProductAddonCategory and are editable in the Console.

Variant max is intentionally not exposed. The Console has no field for it and the column defaults to 1, so publishing it would cap every multi-select variant at a single choice.

Tests

• ProductResourceTest asserts both fields on public and internal payloads, plus the unset case (optional, no limit).
• Full suite: 580 tests, 0 failures. All changed lines covered.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (62af547) to head (2aeffa6).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##                main      #118   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity      2282      2283    +1     
===========================================
  Files            182       182           
  Lines           9082      9086    +4     
===========================================
+ Hits            9082      9086    +4     

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

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