Fix server-side API sorting

[munrojm]

Ensures the sorting stage in the query pipeline is properly ordered with the unique key last.

[codecov]

Codecov Report

Base: 88.32% // Head: 88.34% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (4c4995a) compared to base (cbbe72e).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #766      +/-   ##
==========================================
+ Coverage   88.32%   88.34%   +0.01%     
==========================================
  Files          41       41              
  Lines        3179     3182       +3     
==========================================
+ Hits         2808     2811       +3     
  Misses        371      371              

Impacted Files	Coverage Δ
src/maggma/api/resource/post_resource.py	90.14% <100.00%> (+0.14%)	⬆️
src/maggma/api/resource/read_resource.py	86.44% <100.00%> (+0.11%)	⬆️
src/maggma/api/resource/submission.py	82.22% <100.00%> (+0.13%)	⬆️

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