events: fix Event.log_deprecation not checking that cause is a string (cherry-pick #22598 to version-2026.5)

[authentik-automation]

Cherry-pick of #22598 to version-2026.5 branch.

Original PR: #22598
Original Author: @BeryJu
Cherry-picked commit: 7f31c4c

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.29%. Comparing base (edec3f7) to head (78030db).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@               Coverage Diff               @@
##           version-2026.5   #22683   +/-   ##
===============================================
  Coverage           93.29%   93.29%           
===============================================
  Files                1033     1033           
  Lines               60090    60098    +8     
  Branches              400      400           
===============================================
+ Hits                56060    56068    +8     
  Misses               4030     4030           

Flag	Coverage Δ
conformance	36.59% <0.00%> (-0.01%)	⬇️
e2e	41.83% <0.00%> (-0.01%)	⬇️
integration	33.03% <0.00%> (-0.01%)	⬇️
rust	0.00% <ø> (ø)
unit	92.22% <100.00%> (+1.72%)	⬆️
unit-migrate	92.27% <100.00%> (+<0.01%)	⬆️

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