Add cluster auth preflight check to run-benchmark.sh (kroxylicious#3751)

Fail fast with a clear error message when kubectl is not authenticated,
instead of failing deep into the benchmark run with cryptic API errors.

Assisted-by: Claude Opus 4.6 <noreply@anthropic.com>

Signed-off-by: Sam Barker <sam@quadrocket.co.uk>

Author: SamBarker

kroxylicious-openmessaging-benchmarks/scripts/run-benchmark.sh

@@ -169,6 +169,12 @@ if [[ -n "${CLUSTER_OVERRIDES}" && ! -f "${CLUSTER_OVERRIDES}" ]]; then
     exit 1
 fi
 
+# Preflight: verify we can talk to the cluster
+if ! kubectl auth can-i get pods -n "${NAMESPACE}" &>/dev/null; then
+    echo "Error: not authenticated to cluster — check kubectl/oc login" >&2
+    exit 1
+fi
+
 METRICS_PID=""
 LOGS_PID=""