fix logic and tests, update docker CI file

app/Services/FeeCalculationService.php

@@ -0,0 +1,19 @@
+<?php
+
+namespace App\Services;
+
+use App\Services\Fees\FeeRefundCalculatorService;
+
+class FeeCalculationService
+{
+    public function __construct(
+        private FeeRefundCalculatorService $refundCalculator
+    ) {
+    }
+
+    public function calculateRefund(array $students, int $parentId): float
+    {
+        $result = $this->refundCalculator->calculateRefund($students, $parentId);
+        return (float) ($result['refund_amount'] ?? 0);
+    }
+}