Fix units setup in vector projection Poisson test

[ss2098]

Summary

This PR fixes test_vector_projection_after_poisson_with_units() by setting model reference quantities before creating meshes and MeshVariable objects with physical units.

The test previously failed in strict units mode with:

ValueError: Strict units mode: Cannot create variable 'T' with units='kelvin' when model has no reference quantities.

What changed

Added reference quantity setup at the start of test_vector_projection_after_poisson_with_units():

uw.reset_default_model()
model = uw.get_default_model()
model.set_reference_quantities(
    domain_depth=uw.quantity(1.0, "m"),
    plate_velocity=uw.quantity(1.0, "m/s"),
    mantle_viscosity=uw.quantity(1.0, "Pa*s"),
    temperature_difference=uw.quantity(1.0, "K"),
)

Rationale

Strict units mode requires model reference quantities to be configured before creating variables with units. This updates the test setup to match the current units-system behavior. The Poisson solve and vector projection logic are unchanged.

Testing

Ran:

pixi run python tests/test_1450_poisson_vector_projection.py

Output:

Structured box element resolution 16 16
✓ test_vector_projection_after_poisson passed
✓ Units system active with automatic non-dimensionalization
Structured box element resolution 16 16
✓ test_vector_projection_after_poisson_with_units passed
Structured box element resolution 16 16
✓ test_multiple_solvers_sequence passed

✓ All tests passed!