Add Mutation Testing for improved code quality

[ManukMinasyan]

Overview

Implement mutation testing to improve test suite quality and catch edge cases.

Tasks

• Set up mutation testing framework (Infection PHP)
• Run baseline mutation tests
• Analyze mutation score
• Improve tests to kill mutations
• Integrate into CI/CD
• Set mutation score targets

Success Criteria

• Mutation testing integrated
• Baseline mutation score established
• CI/CD runs mutation tests
• Documentation for running mutation tests
• Target mutation score >70%