test(e2e): cover computeplane placement routing (placement: ComputePlane) #4715

#3369 described a placement: ComputePlane scenario: an application scheduled onto the ComputePlane cluster rather than the management cluster. #4713 covers the module itself (install, addons, kubeconfig visibility, teardown order) but not placement routing, and closing #3369 removes the only place the scenario was written down. Fix shape: extend the computeplane suite with an application that requests ComputePlane placement, and assert it lands in the ComputePlane cluster (its workload in the tenant cluster, nothing in the management namespace) and is removed with it.