Test ros2 lifecycle output streams

[Miko997]

Description

Verify that ros2 lifecycle set writes command errors to stderr and successful transitions to stdout. Also require the unused stream to remain empty so regressions cannot pass through combined process output.

Addresses #484

Is this user-facing behavior change?

No.

Did you use Generative AI?

No.

Additional Information

Validation:

• python3 -m compileall -q ros2lifecycle/test/test_cli.py
• git diff --check origin/rolling...HEAD

The full ROS integration test was not run locally because this environment does not contain ROS 2 or colcon.

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[Miko997]

Validated the current head 6b6e8e27e7b0919df85610f9fb3c1af507767ff5 in the official ros:rolling-ros-base container using rmw_fastrtps_cpp. Both packages built successfully, and the ros2lifecycle test suite passed: 5 tests, 0 errors, 0 failures, 0 skipped.