test: improve config test coverage

[jescalada]

This PR increases test coverage for src/config by testing many of the missing logic branches and helper functions.

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[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 63.28%. Comparing base (a3834ab) to head (c69427b).
Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
src/config/ConfigLoader.ts	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1032      +/-   ##
==========================================
+ Coverage   60.15%   63.28%   +3.12%     
==========================================
  Files          53       53              
  Lines        2196     2187       -9     
  Branches      250      245       -5     
==========================================
+ Hits         1321     1384      +63     
+ Misses        840      774      -66     
+ Partials       35       29       -6     

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[JamieSlome]

LGTM! 🍰