Issues with the box_recursion Exercise of rust_tests

[BenaliOssama]

** Wrong Return Type in last_worker**

Problem:
The instructions require this function signature:

pub fn last_worker(&self) -> Option<(String, Role)>

But the provided solution uses:

pub fn last_worker(&self) -> Option<(String, String)>

This causes a mismatch with the tests, which expect a Role enum, not a String.

Fix:
Implement the Role enum and use it in both the Worker struct and the return type of last_worker.

---

3. Missing Role Enum

Problem:
Tests expect values like Role::CEO, Role::Manager, and Role::Worker.
The provided solution stores roles as String instead, which is incorrect.

Fix:
Define the Role enum as follows:

#[derive(Debug, PartialEq)]
pub enum Role {
    CEO,
    Manager,
    Worker,
}

impl From<&str> for Role {
    fn from(s: &str) -> Self {
        match s {
            "CEO" => Role::CEO,
            "Manager" => Role::Manager,
            _ => Role::Worker,
        }
    }
}

Then update the Worker struct to use Role:

pub struct Worker {
    pub role: Role,
    pub name: String,
    pub next: Link,
}