add namespace argument to node_container in composition_launch.xml (backport #6029)

[mergify]

Description

This adds a namespace argument to node_container in the Using ROS 2 launch to launch composable nodes how-to guide.

In the current form, executing the example launchfile results in

[INFO] [launch]: All log files can be found below /home/ott/.ros/log/2025-11-18-14-24-35-136782-ade-863871
[INFO] [launch]: Default logging verbosity is set to INFO
[ERROR] [launch]: Caught exception in launch (see debug for traceback): Caught multiple exceptions when trying to load file of format [xml]:
 - TypeError: ComposableNodeContainer.__init__() missing 1 required keyword-only argument: 'namespace'
 - SyntaxError: invalid syntax (test.launch.xml, line 1)

This seems to have been mandatory since 2020: ros2/launch_ros#189

Did you use Generative AI?

No.

Additional Information

Tested on jazzy, probably applicable to everything >Foxy

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This is an automatic backport of pull request #6029 done by [Mergify](https://mergify.com).

[github-actions]

HTML artifacts: https://github.com/ros2/ros2_documentation/actions/runs/19510833600/artifacts/4617839370.

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1. Click on the above link to download the artifacts archive
2. Extract it
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