Vehicle.java

## 🎯 Purpose
Base class for all vehicle types.

## 🛠️ Tasks
- [ ] Create fields: id, model, ratePerDay, isAvailable.
- [ ] Define abstract method `getType()`.
- [ ] Include a `getDisplayName()` method.

## ✅ Acceptance Criteria
- Can be extended by Car and Motorcycle.
- Methods behave as expected.
