Add mutation testing for test quality validation #59

Priority: Low 🟢
From PRs: #38

Description

Implement mutation testing to verify test effectiveness.

Tasks

• Implement mutmut or similar tool to verify test effectiveness
• Validate that test quality matches coverage percentages
• Add mutation testing to CI pipeline for critical modules

Related PRs

• PR test: reorganize tests into three-tier structure and improve coverage to 70% #38: Test quality validation recommendations