Add unit testing infrastructure with pytest

## Summary - Added pytest and pytest-cov as development dependencies - Created comprehensive unit tests for the completions module - Added testing section to README with instructions - Test coverage includes validation of the undefined variants bug fix ## Test plan - [x] Run `pytest` to execute all tests - [x] Run `pytest --cov=mpbuild --cov-report=term-missing` to check coverage - [x] All tests pass successfully 🤖 Generated with [Claude Code](https://claude.ai/code)