fix(robot): pin pytest to 7.4.* so apt launch_pytest stays compatible

The builder-stage pip block pulled pytest >=8 transitively into /usr/local (copied
into the runtime image), shadowing Jazzy's apt python3-pytest 7.4. pytest 8 removed
the `path` argument from pytest_pycollect_makemodule, which apt's launch_pytest plugin
still declares — so every pytest invocation in the robot container aborted at plugin
registration. This broke `colcon test` for ament_python packages (e.g.
lidar_point_cloud_filter in test_colcon_test_robot), while ament_cmake gtest packages
were unaffected.

Pin pytest to Jazzy's version so the container is internally consistent and
launch_testing / launch_pytest remain usable for future launch-based tests. The test
runner (tests/docker) is a separate interpreter and keeps its newer pytest.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: JohnYanxinLiu

robot/docker/Dockerfile.robot

@@ -131,6 +131,7 @@ RUN if echo "$BASE_IMAGE" | grep -qE "(nvidia|l4t)" && [ "${SKIP_TENSORRT}" != "
 # Note: numpy>=1.26 required for Python 3.12 compatibility
 # Using --ignore-installed to avoid conflicts with system packages
 RUN pip3 install --break-system-packages --ignore-installed \
+  "pytest==7.4.*" \
   empy==3.3.4 \
   future \
   lxml \