Add initial_value checks to state interface tests (backport #765)

[mergify]

This PR adds a test to verify that the slider_to_cart joint is present in
/joint_states before the velocity example starts moving the robot.

This addresses the part of issue #764 related to checking initial_value
availability before motion. The test does not assert the numeric value of the
initial position, since /joint_states may publish an uninitialized value in
the first cycles of simulation. Instead, it ensures that the joint is present
and published before the controller starts sending commands.

All tests pass locally, including flake8.

---

This is an automatic backport of pull request #765 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 5c63578 has failed:

On branch mergify/bp/humble/pr-765
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit 5c63578.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   gz_ros2_control_demos/launch/pendulum_example_effort.launch.py
	modified:   gz_ros2_control_demos/urdf/test_pendulum_effort.xacro.urdf
	modified:   gz_ros2_control_tests/tests/CMakeLists.txt
	modified:   gz_ros2_control_tests/tests/effort_test.py
	new file:   gz_ros2_control_tests/tests/pendulum_effort_test.py
	modified:   gz_ros2_control_tests/tests/position_test.py
	modified:   gz_ros2_control_tests/tests/velocity_test.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   gz_ros2_control_tests/tests/ft_sensor_test.py
	both modified:   gz_ros2_control_tests/tests/position_test_backward_compatibility.py

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally