chore(kubeclient): tidy indexers and add tests

[hiddeco]

Backfills some tests and other tidying around indexers which I happened to run into.

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[codecov]

Codecov Report

Attention: Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 47.63%. Comparing base (9101f2d) to head (35562db).
Report is 1 commits behind head on main.

Files	Patch %	Lines
internal/kubeclient/indexer.go	62.50%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2300      +/-   ##
==========================================
+ Coverage   47.24%   47.63%   +0.39%     
==========================================
  Files         244      244              
  Lines       17374    17379       +5     
==========================================
+ Hits         8208     8279      +71     
+ Misses       8753     8689      -64     
+ Partials      413      411       -2     

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