Allow unnamed composable nodes in XML and YAML frontends

[Old-Ding]

Summary

• make the regular composable node name attribute optional in the shared XML/YAML parser, matching the Python ComposableNode API
• omit the constructor argument when no name is provided so the component plugin can use its default name
• allow unnamed composable lifecycle nodes when autostart is omitted or explicitly disabled
• continue requiring a lifecycle node name when autostart is enabled or substitution-driven, because lifecycle event handling starts before the container returns the actual default name
• add XML and YAML regression coverage for the supported and rejected cases

Testing

• reproduced the regular composable node missing-name AttributeError with both real launch frontends
• reproduced the original unconditional lifecycle-name rejection with autostart disabled
• focused real frontend probe: XML/YAML omitted and explicit-false lifecycle autostart cases accepted; both true cases rejected
• ament_flake8 on both follow-up files
• ament_pep257 on both follow-up files
• ament_copyright on both follow-up files
• python -m py_compile on both follow-up files
• git diff --check

The repository-native frontend test was not run locally because this Windows environment does not have the compiled ROS message packages or rclpy. The focused probes used the real launch_xml and launch_yaml frontends and replaced only the unavailable ROS import boundary.

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