[GRASS-git] [OSGeo/grass] ab6f35: r3.cross.rast: add test file (#6117)

[Jayneel Shah]

  Branch: refs/heads/main
  Home:   https://github.com/OSGeo/grass
  Commit: ab6f35014ec46c07f76b93fa6268ef9e4fa4b3b2
      https://github.com/OSGeo/grass/commit/ab6f35014ec46c07f76b93fa6268ef9e4fa4b3b2
  Author: Jayneel Shah <80264736+jayneel-shah18 at users.noreply.github.com>
  Date:   2025-07-26 (Sat, 26 Jul 2025)

  Changed paths:
    A raster3d/r3.cross.rast/testsuite/test_r3_cross_rast.py

  Log Message:
  -----------
  r3.cross.rast: add test file (#6117)

This PR introduces a regression test suite for the `r3.cross.rast` GRASS module. The tests are designed to validate the correctness and stability of 2D cross-section extraction from 3D voxel data using various elevation maps under controlled region settings.

### Tests Included:
* **test\_cross\_section\_at\_flat\_plane**: Verifies that extracting a cross-section at a constant elevation (z = 5) returns the correct voxel values. Serves as a baseline test for fixed Z-plane interpolation.

* **test\_cross\_section\_with\_diagonal\_elevation**: Tests cross-section generation with a sloped elevation surface defined as `(row() + col()) / 2`. Confirms statistics based on expected voxel data.

* **test\_elevation\_above\_voxel\_returns\_nulls**: Ensures that when elevation values lie above the 3D raster extent, the resulting 2D output is completely NULL. This validates correct handling of out-of-bound Z queries.

> **Note:** This test uses `r.univar -g` instead of `format=json` because the JSON output returns `None` when the entire map is NULL. If there is another way to detect this condition without using the deprecated `-g` flag, I will update the implementation accordingly.

* **test\_null\_cells\_in\_elevation\_propagate**: Confirms that NULL values in the input elevation raster result in NULL values in the corresponding output cells. This validates elevation NULL handling logic in the module.



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