Fix pressure grad

[chengcli]

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[Copilot]

Pull request overview

This PR fixes the radial pressure-gradient handling in curvilinear coordinate source terms so that providing a face-pressure field does not inadvertently cancel the physical pressure gradient in the x1-momentum equation.

Changes:

• Adjust radial momentum source terms in gnomonic-equiangle and spherical-polar coordinates to preserve the face-pressure gradient.
• Add regression tests to ensure x1-momentum preserves a prescribed face-pressure gradient in both coordinate systems.
• Apply minor C++ pointer-style formatting updates in the coordinate test file.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
tests/test_coordinate.cpp	Adds regression tests validating that face-pressure gradients are preserved in radial momentum updates (gnomonic-equiangle and spherical-polar).
src/coord/spherical_polar.cpp	Subtracts the explicit face-pressure gradient from the geometric pressure source term to avoid canceling the gradient already present in flux divergence.
src/coord/gnomonic_equiangle.cpp	Mirrors the spherical-polar fix for gnomonic-equiangle by subtracting the explicit face-pressure gradient from the radial source term.

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