fix(ci): harden the CA refresh script and add shellcheck to CI

Round-2 review findings, all low severity.

- `curl` now has `--connect-timeout` and `--max-time`, so an unreachable API
  server fails the job instead of hanging the runner indefinitely.

- A guard for a kubeconfig with no cluster entries, or an empty one, with a
  clear message rather than an opaque kubectl error.

- The fingerprint mismatch message now prints both the expected and the
  actual value, which cuts the time to diagnose the next CA rotation.

- `set -euo pipefail` was already there; the script now passes shellcheck
  clean, and a shellcheck step was added to the CI test job so the
  operational scripts get the same treatment as the Python code.

scripts/kube-ca-refresh.sh

@@ -55,16 +55,33 @@ if [ -z "$EXPECTED" ]; then
     exit 1
 fi
 
+# Guard against a kubeconfig with no clusters, or an empty one.
+CLUSTER_COUNT=$(kubectl config view --raw -o jsonpath='{.clusters}' 2>/dev/null \
+    | grep -c '"name"' || true)
+if [ "${CLUSTER_COUNT:-0}" -eq 0 ]; then
+    echo "ERROR: the kubeconfig has no cluster entries." >&2
+    echo "  The KUBEC_CONFIG_BUILDX_NEW_2 secret looks wrong or empty." >&2
+    exit 1
+fi
+
 SERVER=$(kubectl config view --raw -o jsonpath='{.clusters[0].cluster.server}')
 # The cluster entry's NAME - patching the CA into a *new* entry named
 # "default" would leave the original entry (and its stale CA) in use.
 CLUSTER_NAME=$(kubectl config view --raw -o jsonpath='{.clusters[0].name}')
+
+if [ -z "$SERVER" ] || [ -z "$CLUSTER_NAME" ]; then
+    echo "ERROR: could not read the server or cluster name from the kubeconfig." >&2
+    exit 1
+fi
+
 echo "server: $SERVER  cluster entry: $CLUSTER_NAME"
 
 CA_FILE=$(mktemp)
 trap 'rm -f "$CA_FILE"' EXIT
 
-curl -sk "$SERVER/cacerts" -o "$CA_FILE"
+# Timeouts so an unreachable API server fails the job rather than hanging
+# the runner indefinitely.
+curl -sk --connect-timeout 10 --max-time 30 "$SERVER/cacerts" -o "$CA_FILE"
 if [ ! -s "$CA_FILE" ]; then
     echo "ERROR: could not fetch the cluster CA from $SERVER/cacerts" >&2
     exit 1
@@ -73,7 +90,7 @@ fi
 # Normalise both to lowercase hex without separators, so the comparison does
 # not depend on the colon-separated format openssl prints.
 normalise() {
-    tr -d ':' | tr 'A-Z' 'a-z'
+    tr -d ':' | tr '[:upper:]' '[:lower:]'
 }
 
 ACTUAL=$(openssl x509 -in "$CA_FILE" -noout -fingerprint -sha256 | cut -d= -f2 | normalise)
@@ -84,6 +101,8 @@ echo "actual   CA fingerprint: $ACTUAL"
 
 if [ "$ACTUAL" != "$WANTED" ]; then
     echo "ERROR: the cluster CA does not match the pinned fingerprint." >&2
+    echo "  expected: $WANTED" >&2
+    echo "  actual:   $ACTUAL" >&2
     echo "  Refusing to use it.  Either the CA has been rotated and the" >&2
     echo "  pinned fingerprint needs updating, or the connection was" >&2
     echo "  intercepted." >&2