[SPARK-56142] Add worker StatefulSet replica validation to SparkCluster e2e test

### What changes were proposed in this pull request?

This PR adds a worker StatefulSet replica validation to the SparkCluster e2e test. A new assertion file (`spark-cluster-worker-statefulset.yaml`) verifies that the worker StatefulSet's `spec.replicas` and `status.readyReplicas` match the `initWorkers` configuration.

### Why are the changes needed?

The existing SparkCluster e2e test only validates the state transition history (`Submitted` → `RunningHealthy`). It does not verify that the worker StatefulSet is actually created with the correct number of replicas. This gap means the test could pass even if the worker scaling is broken.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Pass the CIs with the newly added validation.

### Was this patch authored or co-authored using generative AI tooling?

Generated-by: Claude Code (Claude Opus 4.6)

Closes #577 from dongjoon-hyun/SPARK-56142.

Authored-by: Dongjoon Hyun <dongjoon@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

Author: dongjoon-hyun

tests/e2e/assertions/spark-cluster/spark-cluster-worker-statefulset.yaml

@@ -0,0 +1,26 @@
+#
+# Licensed to the Apache Software Foundation (ASF) under one or more
+# contributor license agreements.  See the NOTICE file distributed with
+# this work for additional information regarding copyright ownership.
+# The ASF licenses this file to You under the Apache License, Version 2.0
+# (the "License"); you may not use this file except in compliance with
+# the License.  You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+apiVersion: apps/v1
+kind: StatefulSet
+metadata:
+  name: spark-cluster-succeeded-test-worker
+  namespace: ($SPARK_APP_NAMESPACE)
+spec:
+  replicas: 1
+status:
+  readyReplicas: 1

tests/e2e/state-transition/chainsaw-test.yaml

@@ -70,6 +70,12 @@ spec:
             value: default
         timeout: 60s
         file: "../assertions/spark-cluster/spark-cluster-state-transition.yaml"
+    - assert:
+        bindings:
+          - name: SPARK_APP_NAMESPACE
+            value: default
+        timeout: 60s
+        file: "../assertions/spark-cluster/spark-cluster-worker-statefulset.yaml"
     catch:
     - describe:
         apiVersion: spark.apache.org/v1