Fix internal boundary integral MPI test deform API

[ss2098]

This PR updates tests/parallel/test_0765_internal_boundary_integral_mpi.py so the deformed spherical-shell boundary-area MPI test uses the public mesh.deform() API instead of calling the internal Mesh._deform_mesh() method directly.

The previous test path is rejected by the coordinate-mutation capability guard on current development. This keeps the test aligned with the public mesh-deformation API without changing the purpose of the regression test.

Validation:

• pixi run mpirun -np 2 python -m pytest -q --with-mpi tests/parallel/test_0765_internal_boundary_integral_mpi.py::test_deformed_spherical_shell_boundary_area_parallel
• pixi run mpirun -np 4 python -m pytest -q --with-mpi tests/parallel/test_0765_internal_boundary_integral_mpi.py::test_deformed_spherical_shell_boundary_area_parallel
• pixi run mpirun -np 2 python -m pytest -q --with-mpi tests/parallel/test_0765_internal_boundary_integral_mpi.py
• pixi run mpirun -np 4 python -m pytest -q --with-mpi tests/parallel/test_0765_internal_boundary_integral_mpi.py

[lmoresi]

Triage note (2026-07-23): this one-line fix (_deform_mesh → deform in test_0765) landed on development via PR #396 (merged today, with issue #331 closed), so this PR is superseded — the change it proposes is already in. Thanks @ss2098 for spotting it early; recommending the maintainers close this one.

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[lmoresi]

Closing as superseded — the identical fix landed via PR #396 (see comment above).