Add unit test for rangeReader

[gammazero]

• Add unit test
• Simplify retrieve logic

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4c4db87) 73.90% compared to head (77a2e21) 73.97%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #410      +/-   ##
==========================================
+ Coverage   73.90%   73.97%   +0.07%     
==========================================
  Files         149      149              
  Lines        9829     9819      -10     
==========================================
  Hits         7264     7264              
+ Misses       1813     1804       -9     
+ Partials      752      751       -1     

Files	Coverage Δ
handler/file/retrieve.go	64.39% <100.00%> (+1.71%)	⬆️

... and 1 file with indirect coverage changes

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