fix: handle division by zero in divide function

[quantcode-agent]

Summary

• Fixed the division-by-zero bug in src/calculator.ts by adding a guard clause that throws Error("Division by zero") when the divisor is 0
• Added three tests in test/calculator.test.ts covering normal division and the zero-divisor error case

Changes

src/calculator.ts

Added input validation to divide:

export function divide(a: number, b: number): number {
  if (b === 0) {
    throw new Error("Division by zero")
  }
  return a / b
}

test/calculator.test.ts

• Imported divide alongside existing imports
• Added tests: divide(10, 2) === 5, divide(9, 3) === 3, and divide(5, 0) throws "Division by zero"

All 6 tests pass.