fix: include number/2 divisor in FriendlyNumbers sumDivisors

sumDivisors looped 'for (let i = 0; i < number / 2; i++)', which excludes
i === number/2 -- a real divisor of every even number -- so the divisor
sum (and abundancy index) was wrong for even inputs, e.g. sumDivisors(6)
returned 9 instead of 12 and sumDivisors(28) returned 42 instead of 56.
That made FriendlyNumbers misclassify pairs (e.g. FriendlyNumbers(15, 20)
returned true though their abundancy indices differ).

Loop from 1 to number/2 inclusive. Added a test file (the module had none).

Author: Shaked Shlomo

Maths/FriendlyNumbers.js

@@ -29,7 +29,7 @@ function abundancyIndex(number) {
 
 function sumDivisors(number) {
   let runningSumDivisors = number
-  for (let i = 0; i < number / 2; i++) {
+  for (let i = 1; i <= number / 2; i++) {
     if (Number.isInteger(number / i)) {
       runningSumDivisors += i
     }

Maths/test/FriendlyNumbers.test.js

@@ -0,0 +1,21 @@
+import { FriendlyNumbers } from '../FriendlyNumbers'
+
+describe('FriendlyNumbers', () => {
+  it('should return true for the perfect-number pair 6 and 28', () => {
+    expect(FriendlyNumbers(6, 28)).toBe(true)
+  })
+
+  it('should return true for the friendly pair 30 and 140', () => {
+    expect(FriendlyNumbers(30, 140)).toBe(true)
+  })
+
+  it('should return false for numbers with different abundancy', () => {
+    expect(FriendlyNumbers(15, 20)).toBe(false)
+  })
+
+  it('should throw for invalid input', () => {
+    expect(() => FriendlyNumbers(6, 6)).toThrow()
+    expect(() => FriendlyNumbers(0, 5)).toThrow()
+    expect(() => FriendlyNumbers(1.5, 5)).toThrow()
+  })
+})