Handle unknown IIS log fields on expansion

[PrzemyslawKlys]

Summary

• exclude known IIS log fields from the Fields dictionary
• expand unknown fields into top-level properties and omit Fields on -Expand
• test removal of known fields and expansion behaviour

Testing

• dotnet test
• pwsh -NoLogo -NoProfile -Command "Invoke-Pester -CI -Output Detailed"

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https://chatgpt.com/codex/tasks/task_e_68a8d2ee4c74832eb7d447c4572b9002

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.29%. Comparing base (ef42009) to head (220d2ad).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #10      +/-   ##
==========================================
+ Coverage   82.82%   86.29%   +3.46%     
==========================================
  Files           3        3              
  Lines          99      124      +25     
  Branches       16       17       +1     
==========================================
+ Hits           82      107      +25     
  Misses         13       13              
  Partials        4        4              

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