Pass Hessian to trust-region SciPy methods

[Whning0513]

Summary

• pass a Hessian callable to SciPy for trust-region/Newton-CG methods that require it
• compute the Hessian from the wrapped JAX objective and flatten it back into SciPy's expected array shape
• add an isolated regression test that verifies trust-ncg receives hess

Fixes #240.

Testing

• python -m pytest -q tests/scipy_wrappers_hessian_test.py