Implement Integer.radical() to fix ArithmeticError on 0.radical()

[Hapsa21]

Description

Previously, calling .radical() on 0 (e.g., Integer(0).radical()) raised an ArithmeticError because the Integer class lacked its own implementation, falling back to a generic method that attempted to factor zero.

This PR adds a dedicated radical() method to sage.rings.integer.Integer that:

1. Explicitly returns 0 when called on 0.
2. Delegates to self.factor().radical_value() for all other integers, preserving existing behavior.

Fixes

#41222