fix: 修正 BorrowingServiceTest 建構子參數不符（closes #61）

BorrowingService 在新增 MANAGER 角色功能後，建構子新增了
IUserRepository 與 ViolationService 兩個參數，但測試仍使用
舊的 2 參數版本，導致 CI testCompile 失敗。

補上 InMemoryUserRepository，ViolationService 傳 null
（測試情境無需違規偵測邏輯）。

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: JW-Albert

backend/src/test/java/com/vehicle/management/unit/BorrowingServiceTest.java

@@ -6,6 +6,7 @@
 import com.vehicle.management.domain.model.VehicleStatus;
 import com.vehicle.management.domain.role.RoleFactory;
 import com.vehicle.management.repository.inmemory.InMemoryBorrowingRepository;
+import com.vehicle.management.repository.inmemory.InMemoryUserRepository;
 import com.vehicle.management.repository.inmemory.InMemoryVehicleRepository;
 import com.vehicle.management.service.BorrowingService;
 import com.vehicle.management.service.ConflictException;
@@ -32,7 +33,8 @@ class BorrowingServiceTest {
     void setUp() {
         vehicleRepo = new InMemoryVehicleRepository();
         InMemoryBorrowingRepository borrowingRepo = new InMemoryBorrowingRepository();
-        service = new BorrowingService(borrowingRepo, vehicleRepo);
+        InMemoryUserRepository userRepo = new InMemoryUserRepository();
+        service = new BorrowingService(borrowingRepo, vehicleRepo, userRepo, null);
 
         employee = new User(UUID.randomUUID(), "Alice", "alice@example.com",
                 "hash", Set.of(RoleFactory.create("EMPLOYEE")), Instant.now());