fix(test_cli_remapping): use EnableRmwIsolation for zenoh router (backport #588)

[mergify]

Summary

test_cli_remapping fails in the Rci__nightly-zenoh_ubuntu_noble_amd64 CI because its nodes cannot discover each other — the test uses add_launch_test without RMW isolation, so there is no shared network for cross-process node discovery.

Key Changes

• Add EnableRmwIsolation() as the first launch action so the isolated network is set up before any subprocesses are spawned
• Replace env=dict(os.environ) (snapshot taken before isolation runs) with additional_env={'PYTHONUNBUFFERED': '1'} so each subprocess inherits the live environment at spawn time and joins the isolated network
• Add launch_testing_ros and rmw_test_fixture_implementation as test dependencies in package.xml

Breaking Changes

None

Deep Dive

EnableRmwIsolation calls rmw_test_isolation_start(), which starts an in-process Zenoh router on a random port and then sets ZENOH_CONFIG_OVERRIDE=connect/endpoints=[tcp/127.0.0.1:<port>] in the current process environment (line 136–139). Any subprocess that inherits os.environ will have rmw_zenoh_cpp connect to that router and share the same discovery graph.

The original code does env=dict(os.environ) before the launch description executes, so the snapshot is taken before EnableRmwIsolation.execute() runs and ZENOH_CONFIG_OVERRIDE is not yet set. The subprocesses receive a frozen copy of the environment that predates isolation and therefore never connect to the in-process router. Replacing it with additional_env causes the subprocess to inherit the live os.environ at the time it is actually spawned, after isolation has started.

Closes ros2/rmw_zenoh#932

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This is an automatic backport of pull request #588 done by [Mergify](https://mergify.com).