Fix `BoxOSQP` when configured with `fun` and used with pytree-based APIs

[diegoferigo]

The combined usage of Pytree of matrices API and Quadratic function API is currently broken due to a missing operation performed on generic pytrees. This PR updates the operation to be performed with the existing pytree helpers.

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[diegoferigo]

Failures in CI seem unrelated to the changes of this PR. The osqp_test.py, only test affected by the proposed changes, is passing.

[diegoferigo]

Friendly ping @Algue-Rythme, any comment on this tiny PR?