Tidy docstring placement in vector projection Poisson test

[ss2098]

This follow-up tidies the docstring placement in test_vector_projection_after_poisson_with_units() so that the docstring remains the first statement in the function.

No solver or units-system logic is changed.

Tested with:

pixi run python -m pytest tests/test_1450_poisson_vector_projection.py -q

Result:

3 passed in 8.72s

[lmoresi]

Thanks @ss2098 — this is exactly the docstring-placement tidy I flagged on #227. Docstring is now the function's first statement again. Green CI. Merging.