Increase nbconvert and checkpoints coverage

[blink1073]

• Install nbconvert prereqs on Ubuntu to increase coverage
• Add explicit tests for all contents managers - found a bug in AsyncGenericCheckpointsMixin.restore_checkpoint

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 56.25000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 73.83%. Comparing base (44c3742) to head (5b8914d).
Report is 301 commits behind head on main.

Files with missing lines	Patch %	Lines
jupyter_server/services/contents/checkpoints.py	56.25%	3 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1066      +/-   ##
==========================================
+ Coverage   73.09%   73.83%   +0.74%     
==========================================
  Files          64       64              
  Lines        8266     8267       +1     
  Branches     1640     1640              
==========================================
+ Hits         6042     6104      +62     
+ Misses       1819     1749      -70     
- Partials      405      414       +9     

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