Fix AJAX filter for nested relations

[vigo]

Summary

• ensure DALFRelatedFieldAjax sends the source model metadata and field name for nested relations
• add nested models, admin registrations, factories, and fixtures for exercising AJAX list filters through multi-hop relations
• cover the nested AJAX filter with a regression test

Testing

• pytest tests/testproject/testapp/tests.py

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https://chatgpt.com/codex/tasks/task_e_68e3c00538a0832e88fc9a3b4af23d27

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (900f40d) to head (31e13c8).
⚠️ Report is 28 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #19   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines           47        46    -1     
=========================================
- Hits            47        46    -1     

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

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