Bug fix: Open3D mock leak between test_yolo.py and test_lidar.py causes 2 test failures

[Vo1denz]

Closes #571

Problem

Two tests failed when running the full suite:

• test_build_point_cloud
• test_view_point_cloud

Root cause

test_yolo.py stubs open3d with MagicMock at module level (during pytest
collection) to avoid importing the heavy C-extension. The stub persists
in sys.modules, so when perceptionmetrics.utils.lidar imports open3d it
gets the MagicMock. build_point_cloud then returns a MagicMock object
instead of a real PointCloud.

Fix

Three changes:

1. tests/conftest.py — pre-imports real open3d before any test module
   is collected. The if _stub not in sys.modules guard in test_yolo.py
   then skips stubbing it.
2. tests/test_lidar.py — replaced isinstance(obj, o3d.geometry.PointCloud)
   with type(obj).name == "PointCloud" as a secondary fix for
   Open3D 0.19.0 pybind11 compatibility.
3. tests/test_yolo.py — minor cleanup for clarity.

Testing

Full suite: 46 passed, 0 failed (previously 44 passed, 2 failed).