fix: handle division-by-zero bug in calculator

[quantcode-agent]

Summary

• Fixes the division-by-zero bug in src/calculator.ts where divide(x, 0) would return Infinity instead of raising an error
• The divide() function now throws Error("Cannot divide by zero") when the divisor is zero
• Added test cases for normal division and the divide-by-zero error scenario

Assumptions

• The appropriate behavior for division by zero is to throw an Error rather than returning a special value
• The error message "Cannot divide by zero" is descriptive enough for consumers of this function

Testing

All 5 tests pass (bun test).