Fix preferred_dir for sync contents manager

[vidartf]

Also includes a regression test.

[codecov]

Codecov Report

Base: 79.93% // Head: 80.08% // Increases project coverage by +0.14% 🎉

Coverage data is based on head (5ca45f2) compared to base (a6dea1b).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1173      +/-   ##
==========================================
+ Coverage   79.93%   80.08%   +0.14%     
==========================================
  Files          68       68              
  Lines        8124     8153      +29     
  Branches     1601     1606       +5     
==========================================
+ Hits         6494     6529      +35     
+ Misses       1205     1196       -9     
- Partials      425      428       +3     

Impacted Files	Coverage Δ
jupyter_server/services/contents/manager.py	85.39% <100.00%> (+0.09%)	⬆️
jupyter_server/services/sessions/handlers.py	79.20% <0.00%> (-1.95%)	⬇️
jupyter_server/serverapp.py	79.92% <0.00%> (ø)
jupyter_server/utils.py	85.46% <0.00%> (+0.17%)	⬆️
jupyter_server/services/sessions/sessionmanager.py	88.36% <0.00%> (+0.36%)	⬆️
jupyter_server/services/kernels/kernelmanager.py	81.87% <0.00%> (+0.70%)	⬆️
jupyter_server/extension/utils.py	84.44% <0.00%> (+2.39%)	⬆️
jupyter_server/terminal/__init__.py	100.00% <0.00%> (+100.00%)	⬆️
jupyter_server/terminal/handlers.py	100.00% <0.00%> (+100.00%)	⬆️
jupyter_server/terminal/api_handlers.py	100.00% <0.00%> (+100.00%)	⬆️
... and 1 more

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[blink1073]

Thanks!