chore: Extending code coverage

[gromdimon]

Tasks

• Wrote new tests
• Refactored old one

[codecov]

Codecov Report

Merging #89 (f002bc1) into main (88ad05a) will increase coverage by 8.35%.
Report is 5 commits behind head on main.
The diff coverage is 64.89%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #89      +/-   ##
==========================================
+ Coverage   78.25%   86.60%   +8.35%     
==========================================
  Files          83       32      -51     
  Lines        2198      463    -1735     
  Branches      551        0     -551     
==========================================
- Hits         1720      401    -1319     
+ Misses        436       62     -374     
+ Partials       42        0      -42     

Files	Coverage Δ
backend/app/db/session.py	100.00% <ø> (ø)
backend/app/models/user.py	100.00% <100.00%> (ø)
backend/app/schemas/__init__.py	100.00% <100.00%> (ø)
backend/app/schemas/auth.py	100.00% <100.00%> (ø)
backend/app/main.py	92.00% <50.00%> (-3.66%)	⬇️
backend/app/api/api_v1/endpoints/auth.py	80.00% <80.00%> (ø)
backend/app/core/auth.py	73.68% <55.55%> (-6.97%)	⬇️
backend/app/core/config.py	90.00% <66.66%> (-8.28%)	⬇️
backend/app/api/api_v1/api.py	51.21% <41.17%> (-48.79%)	⬇️

... and 53 files with indirect coverage changes