fix: handle division by zero in calculator divide function

Adds a guard clause to throw an Error('Division by zero') when the
divisor is zero, preventing silent NaN/Infinity results.

Adds two new tests covering normal division and the zero-divisor case.

Author: QuantCode Agent

src/calculator.ts

@@ -15,7 +15,7 @@ export function multiply(a: number, b: number): number {
   return a * b
 }
 
-// BUG: Division by zero is not handled
 export function divide(a: number, b: number): number {
+  if (b === 0) throw new Error("Division by zero")
   return a / b
 }

test/calculator.test.ts

@@ -1,7 +1,5 @@
 import { describe, test, expect } from "bun:test"
-import { add, subtract, multiply } from "../src/calculator"
-
-// Note: divide is intentionally not tested — has a known bug
+import { add, subtract, multiply, divide } from "../src/calculator"
 
 describe("calculator", () => {
   test("adds two numbers", () => {
@@ -15,4 +13,12 @@ describe("calculator", () => {
   test("multiplies two numbers", () => {
     expect(multiply(4, 3)).toBe(12)
   })
+
+  test("divides two numbers", () => {
+    expect(divide(10, 2)).toBe(5)
+  })
+
+  test("divide throws on division by zero", () => {
+    expect(() => divide(10, 0)).toThrow("Division by zero")
+  })
 })