fix: handle division by zero in calculator divide function

Throw an Error('Division by zero') when the divisor is zero instead of
returning Infinity/NaN. Add three tests covering normal division, decimal
results, and the zero-divisor error case.

Author: QuantCode Agent

src/calculator.ts

@@ -15,7 +15,9 @@ 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 +13,16 @@ describe("calculator", () => {
   test("multiplies two numbers", () => {
     expect(multiply(4, 3)).toBe(12)
   })
+
+  test("divides two numbers", () => {
+    expect(divide(10, 2)).toBe(5)
+  })
+
+  test("divides resulting in a decimal", () => {
+    expect(divide(7, 2)).toBe(3.5)
+  })
+
+  test("throws on division by zero", () => {
+    expect(() => divide(10, 0)).toThrow("Division by zero")
+  })
 })