ros_launch has no first-class support for Bazel-built nodes

[frneer]

#344 describes this as a known gap. This issue provides a concrete MRE and documents the full set of obstacles.

The natural ROS 2 pattern for launching a node:

launch_ros.actions.Node(package="my_pkg", executable="my_node")

does not work for Bazel-built nodes. They are never installed into an ament prefix, so ROS 2's libexec lookup (<prefix>/lib/my_pkg/my_node) always fails.

What doesn't work

import launch_ros.actions
from launch import LaunchDescription


def generate_launch_description():

    # This is how you'd naturally write it -- but it fails under Bazel because
    # `eg_talker` and `eg_listener` are not installed in any ament prefix.
    return LaunchDescription(
        [
            launch_ros.actions.Node(
                package="ros2_example_apps",
                executable="eg_talker",
            ),
            launch_ros.actions.Node(
                package="ros2_example_apps",
                executable="eg_listener",
            ),
        ]
    )

ros_launch(
    name = "roslaunch_eg_node_problem",
    data = [
        ":eg_listener",
        ":eg_talker",
    ],
    launch_file = "eg_launch_node_problem.py",
    workspace_name = ros2_example_bazel_installed,
)

A workaround

import launch_ros.actions
from launch import LaunchDescription
from python.runfiles import runfiles


def generate_launch_description():
    r = runfiles.Create()
    prefix = "ros2_example_bazel_installed/ros2_example_apps"

    return LaunchDescription(
        [
            launch_ros.actions.Node(
                executable=r.Rlocation(f"{prefix}/eg_talker", source_repo=""),
            ),
            launch_ros.actions.Node(
                executable=r.Rlocation(f"{prefix}/eg_listener", source_repo=""),
            ),
        ]
    )

What it would be good to have

1. No hardcoded runfiles path strings in launch files
2. Build-time validation that referenced executables exist
3. source_repo="" handled transparently, not by the user

Also, the source_repo requirement is a current bug of the current launch file support:

bazel run //ros2_example_apps:roslaunch_eg_py
.
.
.
[INFO] [launch]: All log files can be found below /home/frn/.ros/log/2026-04-07-15-28-43-711973-vader-2564850
[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 [py]:
 - ValueError: /home/frn/drake-ros/ros2_example_bazel_installed/ros2_example_apps/eg_launch.py does not lie under the runfiles root /home/frn/.cache/bazel/_bazel_frn/a779afcb46037298559646c7c2f921d4/execroot/_main/bazel-out/k8-fastbuild/bin/ros2_example_apps/roslaunch_eg_py.runfiles
 - InvalidFrontendLaunchFileError: The launch file may have a syntax error, or its format is unknown

[frneer]

@julianadrianheine

[frneer]

Done in #419