roslibpy (ros extra) never installed in CI, so TestRosTopicValidation is entirely skipped

Found 2026-08-14 while wiring open3d test coverage into CI for #46.

`pyproject.toml`'s `ros` extra (`rosbags`, `roslibpy`, `websockets`) is never installed
anywhere in `.github/workflows/ci.yml` — only `pip install .[dev,tool]` runs. As a result,
the entire `TestRosTopicValidation` class in `tests/test_ros.py` (class-level
`@pytest.mark.skipif(not _roslibpy_importable, reason="roslibpy not installed")`) is silently
skipped on every CI run, including the three open3d-gated pointcloud-publish tests inside it
(`test_publish_pointcloud_uncoloured`, `test_publish_pointcloud_coloured`,
`test_publish_pointcloud_uses_explicit_timestamp`) — installing open3d alone does not unlock
these, since the class-level roslibpy gate short-circuits first.

Verified locally: with `roslibpy` not installed, `pytest tests/test_ros.py -k pointcloud -rs`
reports `SKIPPED ... roslibpy not installed` for all three, regardless of open3d's install
state.

Other `_rosbags_importable`-gated tests in the same file (`TestRosBagsIO` etc.) are presumably
similarly always-skipped, since `rosbags` isn't installed either — not independently verified
here, worth checking as part of the fix.

Fix

Add a CI leg (or a conditional install step, similar to the open3d pattern from #46/#78) that
installs `.[ros]` on at least one matrix leg, so the `ros`-extra-gated tests actually run
somewhere rather than being permanently invisible to CI.

Beginne mit .github/workflows/ci.yml und pyproject.toml, um zu sehen, wie vorhandene Extras und Matrix-Legs installiert werden. Führe pytest tests/test_ros.py -k pointcloud -rs aus und überprüfe anschließend, dass der ROS-extra-Leg die durch roslibpy und rosbags geschützten Tests ausführt, anstatt sie zu überspringen; der CI-Job sollte mit installiertem Extra erfolgreich abgeschlossen werden.