feat: accept array of parent entity classes in cascade deletion rule

[gwdp]

Why

AllowIfInParentCascadeDeletionPrivacyPolicyRule only accepts a single parentEntityClass, but sibling entity classes backed by the same table need separate rules for each. This adds up when you have entities that share a table but aren't in an inheritance chain.

Thread: https://app.graphite.com/github/pr/expo/universe/28900#discussion-IC_kwDOAz9RTM8AAAABJd5cSg

How

parentEntityClass now accepts a single class or an array. Backward compatible — single class works identically.

Test Plan

New test cases for array form. Existing 740 tests pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (4709f53) to head (2f30bdd).
⚠️ Report is 90 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main      #650    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files          110       109     -1     
  Lines        17588     17755   +167     
  Branches      1515       919   -596     
==========================================
+ Hits         17588     17755   +167     

Flag	Coverage Δ
integration	28.60% <0.00%> (+0.25%)	⬆️
unittest	94.53% <100.00%> (-0.67%)	⬇️

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