fix(verify): close stdin for helper commands so incus launch cannot hang#1801

verify-cluster up could hang forever when the caller left stdin open. incus launch reads stdin when it is not a terminal, so a harness that kept stdin attached (an agent shell, a pipeline) waited on a launch that never returned.

Every helper command now gets stdin=DEVNULL by default, through Commands.__call__ and incus.query. The exec and cli passthroughs keep the caller's stdin, because those run interactive commands on a Machine.

Proof. Before the fix, a repro with stdin held open waited 30 s against 0.156 s with stdin closed, and the new unit test failed on its 5 s timeout. After the fix, on real Incus with the caller's stdin held open (< <(sleep 7200)), the bake's incus launch images:ubuntu/24.04 exited 0 in 9.3 s, the run's launch exited 0 in 26.9 s, and a full up --daemon 0.3.0-beta.1 came up in 676.7 s with exit 0.

Checks. core/scripts/verify/test_commands.py covers an open stdin and passing input, and runs in the CI contracts job.