Improved schema email validation

[tobibytes]

Resolves #775

Description

Added a test case for invalid email validation in the chapter and project directories. It includes:

• A new leader-email-invalid.yaml file with an invalid email (test@gmail).
• Updates to chapter_test.py and project_test.py to check for this invalid email and ensure proper error handling.

Motivation

To improve input validation and ensure the system properly handles invalid email addresses.

How Has This Been Tested?

The test cases for invalid emails were executed, and all tests passed successfully.

[arkid15r]

Hi @tobibytes
Thanks for a clean PR!

Please make sure to address the improvement I mentioned in the issue -- we need to validate the email format, but currently it just yells about its length which kind of works, I agree. But I'd like to have a comprehensive solution -- uou need validators.email() for that (see example here)