Test Cases for `Person` Class

1. Constructor

Test Case	Input	Expected Result
Valid input	Name: "Alice", Surname: "Smith", Birthdate: 1990-01-01, Nationality: "British"	Person created successfully
Empty name	Name: "", Surname: "Smith", Birthdate: 1990-01-01	`IllegalArgumentException`: "Name cannot be empty"
Future birthdate	Birthdate: 2030-01-01	`IllegalArgumentException`: "Birthdate cannot be in the future"
Null birthdate	Birthdate: `null`	`IllegalArgumentException`: "Birthdate cannot be null"

2. `isAdult` Method

Test Case	Input (Birthdate)	Expected Result
Exactly 18 years old	Today - 18 years	`true`
Less than 18 years old	Today - 17 years	`false`
More than 18 years old	Today - 19 years	`true`

3. `isEuropean` Method

Test Case	Input (Nationality)	Expected Result
European nationality	"Austrian"	`true`
Non-European nationality	"American"	`false`
Case-insensitivity test	"austrian"	`true`

4. `canEnrollDoctoralProgram` Method

Test Case	Input (Degree)	Expected Result
Doctorate	`Degree.DOCTORATE`	`true`
Master's degree	`Degree.MASTER`	`true`
Undergraduate degree	`Degree.GRADUATE`	`false`
No degree	`Degree.NONE`	`false`

5. `getAge` Method

Test Case	Input (Birthdate)	Expected Result
Recent birthday	Today - 25 years	`25`
Upcoming birthday	Today - 25 years + 1 day	`24`
Leap year birthdate	2000-02-29	Correct age (e.g., `24`)