Add Unit Tests for Expense Service

[MiguelAntonioRS]

Currently, there are no unit tests covering the business logic of the ExpenseService. This could lead to bugs and regression issues in the future.

Suggested Actions:

Write unit tests using JUnit and Mockito.
Cover all CRUD operations for expenses.
Ensure that test cases validate both success and failure scenarios.
Add test coverage for validation logic (e.g., negative amounts, invalid categories).