test: Fix flaky CustomEventWireModelTests

[tippmar-nr]

Updated CustomEvents_UserAttributes_AllAttributeTypesSerializeCorrectly() (introduced yesterday) to make sure we use the same Jsonconverter for the "expected" serialization result. There was a slight difference in how DateTime values were being serialized without this change.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.19%. Comparing base (80d4c76) to head (b19622d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2978   +/-   ##
=======================================
  Coverage   82.18%   82.19%           
=======================================
  Files         473      473           
  Lines       30364    30364           
  Branches     3388     3388           
=======================================
+ Hits        24955    24957    +2     
+ Misses       4609     4606    -3     
- Partials      800      801    +1     

Flag	Coverage Δ
Agent	83.17% <ø> (+<0.01%)	⬆️
Profiler	73.13% <ø> (ø)

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see 2 files with indirect coverage changes