Add tests for model mixin

[KengoWada]

Closes #25

Added tests for TypesenseModelMixin

Changes made

• Added test settings
• Added test models and collections
• Added Django test runner
• Fixed TypesenseCollection delete method to convert object ids to int
• Updated CONTRIBUTIONS.md to reflect changes in tests.

[KengoWada]

Hey @EricOuma could you please approve the workflows for this PR. Thanks

[codecov-commenter]

Codecov Report

Attention: 17 lines in your changes are missing coverage. Please review.

Comparison is base (4968cd7) 100.00% compared to head (13b668e) 59.62%.

Additional details and impacted files

@@             Coverage Diff              @@
##              main      #27       +/-   ##
============================================
- Coverage   100.00%   59.62%   -40.38%     
============================================
  Files            1       10        +9     
  Lines           12      426      +414     
============================================
+ Hits            12      254      +242     
- Misses           0      172      +172     

Files	Coverage Δ
django_typesense/collections.py	59.06% <55.26%> (ø)

... and 10 files with indirect coverage changes

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[KengoWada]

I'll be closing this PR because it contains unsigned commits.