Add mutation testing for contract validation logic to quantify test suite effectiveness

[ALLEN-AYODEJI]

Description

The test suite has 5 tests covering the contract. Mutation testing determines test quality by introducing code changes (mutations) and checking if tests detect them. This reveals untested code paths and weak assertions. The state machine validation logic is a high-value target for mutation testing.

Requirements and context

• Security: Standard security review required for state-mutating changes
• Testing: Full test suite must pass; new tests for changed behavior
• Documentation: Update doc comments and README if user-facing

Suggested execution

Branch: fix/feat/test-mutation-testing

Implement changes

• Install cargo-mutants or similar mutation testing tool
• Configure mutation targets: focus on lib.rs validation logic, exclude events.rs and storage.rs
• Run mutation testing and generate a report showing which mutations survived (untested code)
• Add tests for any surviving mutations to improve coverage

Test and commit

Run cargo mutants and verify mutation score. Target: >80% mutation kill rate. Document surviving mutations and add tests.

Example commit message

feat(test): add mutation testing for contract validation and state machine logic

Guidelines

• Open a PR against master
• Keep PR focused on this single concern
• Run cargo test or npm test before review
• Ensure no lint or typecheck warnings