fix: handle division by zero in calculator

[quantcode-agent]

Summary

• Added a guard clause in src/calculator.ts divide function that throws Error("Division by zero") when the divisor is zero
• Added test cases in test/calculator.test.ts for normal division and division-by-zero error handling
• All 5 tests pass

Assumptions

• Throwing an Error is the appropriate way to handle division by zero (rather than returning Infinity, NaN, or a result object)
• The error message "Division by zero" is descriptive enough for consumers