Split base_setup tests by feature area

[codeforester]

Summary

• Split the oversized base_setup test module into focused feature-area test files.
• Added shared base_setup.tests.helpers for common test utilities.
• Removed the broad too-many-lines / too-many-public-methods pylint disable by deleting the monolithic test_engine.py.
• Updated contributor guidance so larger Python packages can use focused test_*.py modules.

Validation

• PYTHONPATH=lib/python:cli/python /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest cli/python/base_setup/tests
• PYTHONPATH=lib/python:cli/python /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest
• git ls-files -z '*.py' | xargs -0 env PYTHONPATH=lib/python:cli/python /Users/rameshhp/.base.d/base/.venv/bin/python -m pylint --rcfile=.pylintrc
• git diff --check

Closes #289