fix: include number/2 divisor in FriendlyNumbers sumDivisors

[shaked-shlomo]

Bug

Maths/FriendlyNumbers.js's sumDivisors uses an exclusive upper bound:

for (let i = 0; i < number / 2; i++) {

This excludes i === number / 2, which is a divisor of every even number, so the sum of divisors (and therefore the abundancy index σ(n)/n) is wrong for even inputs:

• sumDivisors(6) returns 9 instead of 12
• sumDivisors(28) returns 42 instead of 56

Because the error scales both numbers similarly it sometimes cancels out (the classic FriendlyNumbers(6, 28) still returns true by luck), but it produces wrong classifications in general — e.g. FriendlyNumbers(15, 20) returns true even though σ(15)/15 = 1.6 ≠ σ(20)/20 = 2.1.

Fix

Loop from 1 to number / 2 inclusive:

for (let i = 1; i <= number / 2; i++) {

(Starting at 1 also drops the harmless number / 0 = Infinity check the old i = 0 start relied on.)

After the fix: sumDivisors(6) = 12, sumDivisors(28) = 56, FriendlyNumbers(6, 28) = true, FriendlyNumbers(30, 140) = true, FriendlyNumbers(15, 20) = false.

The module had no test file, so I added Maths/test/FriendlyNumbers.test.js covering friendly pairs, a non-friendly pair, and invalid input.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.11%. Comparing base (5c39e87) to head (3dd22dd).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1904      +/-   ##
==========================================
+ Coverage   85.91%   86.11%   +0.19%     
==========================================
  Files         379      379              
  Lines       19778    19778              
  Branches     3016     3024       +8     
==========================================
+ Hits        16993    17031      +38     
+ Misses       2785     2747      -38     

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