Fix HeightmapShape visual/collision mismatch (ODE)

[jslee02]

Fixes #1658

HeightmapShape visual rendering and collision geometry were misaligned (most visible with ODE heightfields). This series aligns the visual mesh and collision origin, and tightens heightmap bounds handling.

• Center heightmap render vertices around the origin and align the ODE heightfield transform so visual/collision match.
• Compute HeightmapShape bounding box using sample spacing ((N-1) * scale) and add a regression test.
• Fix ODE heightfield AABB bounds for scale.z < 1 by passing unscaled min/max heights; add an ODE regression test.
• Update the gz-physics integration patch to compute heightmap spacing using (vertSize-1) so Gazebo Physics tests match DART's heightmap semantics.

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Before creating a pull request

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• [x] Add unit tests for new functionality
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