test: cover mz leaked mutation survivors

[qartik]

Summary

• add exact malformed-signature coverage for __quantum__qis__mz_leaked__body return width and parameter type checks
• extract and test the defensive mz_leaked operand-shape guard
• remove a redundant function-body filter from result-slot validation that produced an equivalent mutation survivor

Original failures

The failing mutants job in https://github.com/Quantinuum/qir-qis/actions/runs/24985659961 reported two missed mutations:

• src/lib.rs:788:48: replace && with || in aux::handle_mz_leaked_call
• src/lib.rs:798:13: delete match arm [_, _] in aux::handle_mz_leaked_call

Validation

• make lint
• make test
• make mutants

[Copilot]

Pull request overview

This PR strengthens mutation-test coverage and hardens malformed-call handling around __quantum__qis__mz_leaked__body, while simplifying result-slot validation by removing a redundant filter.

Changes:

• Add precise malformed-signature tests for mz_leaked (wrong return width, non-pointer parameter).
• Extract mz_leaked’s operand-shape validation into a dedicated helper and add a focused unit test for it.
• Simplify validate_result_slot_usage by iterating all module functions (dropping a redundant “has basic blocks” filter).

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.