feat: implement GasCar fuel logic

src/main/java/org/example/model/GasCar.java

@@ -0,0 +1,33 @@
+package org.example.model;
+
+/*
+ * GasCar extends Car.
+ * Inherits Serviceable from Car.
+ *
+ * Service logic for GasCar:
+ * - needsService(): return true if mileage > 12000
+ * - performService():
+ *     Print "Engine oil changed"
+ *     Reset fuelLevel to 100
+ */
+
+public class GasCar extends Car {
+
+    private double fuelLevel;
+    private String fuelType;
+
+    public GasCar(String brand, String model, int year,
+                  double mileage, int seats,
+                  double fuelLevel, String fuelType) {
+
+        super(brand, model, year, mileage, seats);
+        this.fuelLevel = fuelLevel;
+        this.fuelType = fuelType;
+    }
+
+    // TODO: Override getEnergyType() → return fuelType + " (Gas)"
+
+    // TODO: Implement refuel(double amount)
+    // Validate amount
+    // Cap at 100
+}