Add headcode validation and test

src/inputValidation/inputValidation.ts

@@ -67,4 +67,15 @@ export function validateReasonCode(code: unknown): boolean {
 
     // If it's neither a number nor a string, throw an error
     throw new ValidationError("Invalid input: Reason code must be a number or a string of three digits.");
+}
+
+export function validateHeadcode(headcode: unknown): boolean {
+    if (typeof headcode !== "string") {
+        throw new ValidationError("Invalid input: Headcode must be a string");
+    }
+
+    if (!/^\d[A-Za-z]\d{2}$/.test(headcode)) {
+        throw new ValidationError("Invalid input: Headcode must be in the format dLdd where d=digit and L=letter");
+    }
+    return true;
 }