Added improved validation and tests to backend models

[kiancross]

No description provided.

[codecov]

Codecov Report

Merging #269 (5a9aa62) into master (63d00e9) will decrease coverage by 1.00%.
The diff coverage is 57.83%.

@@            Coverage Diff             @@
##           master     #269      +/-   ##
==========================================
- Coverage   75.33%   74.33%   -1.01%     
==========================================
  Files         136      137       +1     
  Lines        1946     1987      +41     
  Branches      274      282       +8     
==========================================
+ Hits         1466     1477      +11     
- Misses        480      510      +30     

Flag	Coverage Δ
backend-core	90.50% <78.26%> (-3.50%)	⬇️
backend-federation-client	77.27% <0.00%> (-5.75%)	⬇️
backend-federation-server	71.81% <50.00%> (-0.49%)	⬇️
backend-internal-server	56.02% <42.85%> (-0.36%)	⬇️
backend-ml	92.11% <ø> (ø)
frontend	71.17% <60.52%> (-0.75%)	⬇️
shared	100.00% <ø> (ø)

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