Fix BulkLoader indexing with Stringable primary keys

[roxblnfk]

🔍 What was changed

Fix BulkLoader crash when primary key uses Stringable typecast without uncast

When a typecast handler implements only CastableInterface (no UncastableInterface), mapper->uncast() returns PK values as Stringable objects. BulkLoader::indexEntity() rejected them via is_scalar() check.

Now both indexEntity() and getEntity() cast Stringable objects to string for array key indexing.

📝 Checklist

• How was this tested:
  • Unit tests added

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.80%. Comparing base (2c06029) to head (8f3fbb3).
⚠️ Report is 1 commits behind head on 2.x.

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x     #557      +/-   ##
============================================
+ Coverage     91.75%   91.80%   +0.04%     
- Complexity     1965     1966       +1     
============================================
  Files           131      131              
  Lines          5036     5039       +3     
============================================
+ Hits           4621     4626       +5     
+ Misses          415      413       -2     

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