tasks: add pre_delete for TasksModel to avoid OOM when deleting object with many tasks linked (cherry-pick #23664 to version-2026.5)

[authentik-automation]

Cherry-pick of #23664 to version-2026.5 branch.

Original PR: #23664
Original Author: @rissson
Cherry-picked commit: 2b58939

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.26%. Comparing base (e329e2e) to head (b025afb).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@                Coverage Diff                 @@
##           version-2026.5   #23666      +/-   ##
==================================================
- Coverage           93.27%   93.26%   -0.02%     
==================================================
  Files                1037     1037              
  Lines               60419    60427       +8     
  Branches              400      400              
==================================================
- Hits                56358    56356       -2     
- Misses               4061     4071      +10     

Flag	Coverage Δ
conformance	36.45% <77.77%> (+<0.01%)	⬆️
e2e	41.68% <100.00%> (+0.03%)	⬆️
integration	32.87% <77.77%> (+<0.01%)	⬆️
rust	0.00% <ø> (ø)
unit	92.20% <100.00%> (+<0.01%)	⬆️
unit-migrate	92.21% <100.00%> (-0.02%)	⬇️

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