Leap Year endpoint crashes on valid input

[YuvalG724]

Bug: The /leap_year POST endpoint crashes when sending a valid year in JSON.

Steps to Reproduce:

1. Run the server: node app.js
2. Send POST request:
   {
   "number": 2024
   }
3. Observe server crash

Expected Behavior:
The server should respond with:
{
"message": "The given year is a leap year"
}

Actual Behavior:
The server throws an error: TypeError: res.status is not a function