fix: unit tests and slight refactor

[kaloster]

Added 2 unit tests:

test_uns_no_metadata_get - for dataset without uns array or spatial
test_uns_metadata_get - for dataset with uns array and spatial

Slight refactor to extract metadata helper functions to a dedicated utils file

[codecov]

Codecov Report

Attention: Patch coverage is 94.87179% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 77.27%. Comparing base (ffd7281) to head (b095100).

Files	Patch %	Lines
server/common/utils/uns.py	93.10%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #877      +/-   ##
==========================================
+ Coverage   76.58%   77.27%   +0.69%     
==========================================
  Files          88       90       +2     
  Lines        7020     7024       +4     
==========================================
+ Hits         5376     5428      +52     
+ Misses       1644     1596      -48     

Flag	Coverage Δ
frontend	77.27% <94.87%> (+0.69%)	⬆️
javascript	77.27% <94.87%> (+0.69%)	⬆️
unitTest	77.27% <94.87%> (+0.69%)	⬆️

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[tihuan]

Thanks so much for adding the tests, Ronen! Some discussion comments PTAL thank you!

[tihuan]

Fantastic work, @kaloster 🙏 🔥 !! Thanks so much for the thorough tests!!