test: simplify and deduplicate KubeRay e2e test code

- Unify 4 identical input structs into single KubeRaySpecInput
- Extract waitForRayClusterReady() and waitForRayPodRunning() helpers
- Simplify newRayClusterWithNativeScheduling() to build on newRayClusterUnstructured()
- Move podGVR to package level (was declared locally in 2 functions)
- Simplify Workload/PodGroup lookups from scan to direct Get by name
- Use rayClusterName variable consistently instead of hardcoded strings
- Net reduction of ~141 lines

Author: mboersma