Fix star terrain crash on NumPy 2 (#6910)

# Description

`star_terrain` computed the bar lengths with `np.math.cos` and
`np.math.sin`. `np.math` was an alias for the standard library `math`
module; it was deprecated in NumPy 1.25 and removed in NumPy 2.0. Since
Isaac Lab 3.0 ships NumPy 2, any terrain configuration containing the
star sub-terrain raised `AttributeError: module 'numpy' has no attribute
'math'`, making `MeshStarTerrainCfg` unusable.

Use the standard library `math` module instead. The alias was the same
module object, so the generated geometry is unchanged.

The regression went unnoticed because `ROUGH_TERRAINS_CFG`, the only
config covered by the terrain generator tests, has no star sub-terrain.
Add a test that generates a star-only terrain, with a bar count that
exercises all three branches of the bar-length computation.

> Note: The testcase is written by Claude. I've confirmed the code fix
myself.

Fixes #6909

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## Type of change

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- Bug fix (non-breaking change which fixes an issue)

## Release backport

- [x] <!-- backport-active-release --> Backport this pull request to the
active release branch after it merges into `develop`

## Checklist

- [x] I have read and understood the [contribution
guidelines](https://isaac-sim.github.io/IsaacLab/main/source/refs/contributing.html)
- [ ] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] I have added a changelog fragment under
`source/<pkg>/changelog.d/` for every touched package (do **not** edit
`CHANGELOG.rst` or bump `extension.toml` — CI handles that)
- [ ] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

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Co-authored-by: ooctipus <zhengyuz@nvidia.com>

(cherry picked from commit b44ea65)

Author: j3soon

source/isaaclab/changelog.d/j3soon-star-terrain-numpy2.rst

@@ -0,0 +1,6 @@
+Fixed
+^^^^^
+
+* Fixed :func:`~isaaclab.terrains.trimesh.mesh_terrains.star_terrain` failing with
+  ``AttributeError: module 'numpy' has no attribute 'math'``. The function used the ``np.math``
+  alias, which was removed in NumPy 2.0, and now uses the standard library ``math`` module instead.

source/isaaclab/isaaclab/terrains/trimesh/mesh_terrains.py

@@ -7,6 +7,7 @@
 
 from __future__ import annotations
 
+import math
 from typing import TYPE_CHECKING
 
 import numpy as np
@@ -694,11 +695,11 @@ def star_terrain(
         # length changes since the bar is connected to a square border
         bar_length = cfg.size[0]
         if yaw < 0.25 * np.pi:
-            bar_length /= np.math.cos(yaw)
+            bar_length /= math.cos(yaw)
         elif yaw < 0.75 * np.pi:
-            bar_length /= np.math.sin(yaw)
+            bar_length /= math.sin(yaw)
         else:
-            bar_length /= np.math.cos(np.pi - yaw)
+            bar_length /= math.cos(np.pi - yaw)
         # compute the transform of the bar
         transform[0:3, 0:3] = tf.Rotation.from_euler("z", yaw).as_matrix()
         # add the bar to the mesh

source/isaaclab/test/terrains/test_terrain_generator.py

@@ -10,7 +10,7 @@
 import pytest
 import torch
 
-from isaaclab.terrains import FlatPatchSamplingCfg, TerrainGenerator, TerrainGeneratorCfg
+from isaaclab.terrains import FlatPatchSamplingCfg, MeshStarTerrainCfg, TerrainGenerator, TerrainGeneratorCfg
 from isaaclab.terrains.config.rough import ROUGH_TERRAINS_CFG
 from isaaclab.utils.seed import configure_seed
 
@@ -49,6 +49,42 @@ def test_generation(output_dir):
     assert actualSize[1] == pytest.approx(expectedSizeY)
 
 
+def test_generation_star_terrain():
+    """Generates a star sub-terrain and tests that the resulting mesh has the expected size.
+
+    The star sub-terrain is not part of :obj:`ROUGH_TERRAINS_CFG`, so it needs its own coverage.
+    """
+    # create terrain generator with only the star sub-terrain
+    cfg = TerrainGeneratorCfg(
+        seed=0,
+        size=(8.0, 8.0),
+        num_rows=1,
+        num_cols=1,
+        use_cache=False,
+        sub_terrains={
+            # the number of bars is chosen so that all the branches of the bar-length computation are covered
+            "star": MeshStarTerrainCfg(
+                proportion=1.0,
+                platform_width=1.5,
+                num_bars=5,
+                bar_width_range=(0.5, 1.0),
+                bar_height_range=(0.05, 0.2),
+            )
+        },
+    )
+    terrain_generator = TerrainGenerator(cfg=cfg)
+
+    # get size from mesh bounds
+    bounds = terrain_generator.terrain_mesh.bounds
+    actual_size = abs(bounds[1] - bounds[0])
+
+    # check if the size is as expected
+    assert actual_size[0] == pytest.approx(cfg.size[0] * cfg.num_rows + 2 * cfg.border_width)
+    assert actual_size[1] == pytest.approx(cfg.size[1] * cfg.num_cols + 2 * cfg.border_width)
+    # check the sub-terrain origin is at the center of the terrain
+    assert terrain_generator.terrain_origins.shape == (cfg.num_rows, cfg.num_cols, 3)
+
+
 @pytest.mark.parametrize("use_global_seed", [True, False])
 @pytest.mark.parametrize("seed", [20, 40, 80])
 def test_generation_reproducibility(use_global_seed, seed):