resolve sonarcloud warnings

[christianhelle]

• use isinstance() for direct type comparison
• Specify return types
• Use assertIn instead of assertTrue

[codecov]

Codecov Report

Attention: Patch coverage is 95.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 97.96%. Comparing base (000f7e7) to head (abb1dd2).
Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
src/autofaker/generator.py	0.00%	1 Missing ⚠️
tests/test_create_anonymous_builtins.py	94.73%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #49      +/-   ##
==========================================
- Coverage   97.96%   97.96%   -0.01%     
==========================================
  Files          46       46              
  Lines        2508     2506       -2     
==========================================
- Hits         2457     2455       -2     
  Misses         51       51              

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[sonarqubecloud]

Quality Gate failed

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)

See analysis details on SonarCloud