Weierstrass Method (#12877)

* Add weierstrass_method for approximating complex roots

- Implements Durand-Kerner (Weierstrass) method for polynomial root finding
- Accepts user-defined polynomial function and degree
- Uses random perturbation of complex roots of unity for initial guesses
- Handles validation, overflow clipping, and includes doctest

* Update weierstrass_method.py
* add more tests

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* Update weierstrass_method.py

* Update weierstrass_method.py

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