fix: ensure calculateMean validates input as an array of numbers

Author: MattCarolus10

Sprint-1/implement/mean.js

@@ -5,6 +5,14 @@ function calculateMean(list) {
   if (list.length === 0) {
     throw new Error("calculateMean requires a non-empty array");
   }
+  if (!list.every((item) => typeof item === "number")) {
+    throw new Error("calculateMean requires an array of numbers");
+  }
+  let total = 0;
+  for (const item of list) {
+    total += item;
+  }
+  return total / list.length;
 }
 
 module.exports = calculateMean;