Type annotations do not reflect that Entity.get_attr()'s data_type parameter can be None

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Operating System:

all

ROS version or commit hash:

Rolling

RMW implementation (if applicable):

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RMW Configuration (if applicable):

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Client library (if applicable):

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'ros2 doctor --report' output

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Steps to reproduce issue

Run tests downstream of launch with the assumption that data_type can't be None, e.g., https://github.com/ros2/launch/pull/887#issuecomment-2987111760 for #887.

Expected behavior

Tests pass

Actual behavior

Tests fail if we assume that data_type can't be None, see these test failures for #887:

1. https://ci.ros2.org/job/ci_linux/24014/testReport/junit/test_launch_ros.test.test_launch_ros.frontend/test_component_container/test_launch_component_container_xml/
2. https://ci.ros2.org/job/ci_linux/24014/testReport/junit/test_launch_ros.test.test_launch_ros.frontend/test_node_frontend/test_launch_frontend_xml/

Additional information

See https://github.com/ros2/launch/pull/887#discussion_r2157614696. The type annotation for data_type in launch_xml/launch_yaml's Entity.get_attr() is AllowedTypesType, which does not include None:

1. launch_xml: https://github.com/ros2/launch/blob/6854fae626f2948139e1734e68d712cc1e0def1b/launch_xml/launch_xml/entity.py#L79
2. launch_yaml: https://github.com/ros2/launch/blob/6854fae626f2948139e1734e68d712cc1e0def1b/launch_yaml/launch_yaml/entity.py#L102

However, the documentation in launch's Entity.get_attr() mentions that data_type can be None, in which case it behaves like str (IIUC), which is the default data_type value: https://github.com/ros2/launch/blob/6854fae626f2948139e1734e68d712cc1e0def1b/launch/launch/frontend/entity.py#L89

Interestingly, launch's Entity.get_attr()'s type annotations are kind of incomplete, especially for data_type. They use TargetType, but that does not specify any type, and, critically, it does not specify None/Optional: https://github.com/ros2/launch/blob/6854fae626f2948139e1734e68d712cc1e0def1b/launch/launch/frontend/entity.py#L46-L76