refactor: encapsulate CMS size guardrail in CMSSizeGuardrail

Move the guardrail construction and the reconfigure skip logic into a
dedicated CMSSizeGuardrail class, per review (aba3e2a944). Keeps the
cmsSize >= totalNodes clamp rather than reintroducing the
totalNodes < minCmsSize skip, so the guardrail is not silently bypassed
when the configured threshold exceeds the current cluster size. The
clamp needs no threshold value, so the guard() overload takes only
(totalNodes, cmsSize) and CMSOperations no longer reads the threshold.

src/java/org/apache/cassandra/db/guardrails/CMSSizeGuardrail.java

@@ -0,0 +1,59 @@
+/*
+ * 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.
+ */
+
+package org.apache.cassandra.db.guardrails;
+
+import java.util.function.ToLongFunction;
+
+import org.apache.cassandra.service.ClientState;
+
+import static java.lang.String.format;
+
+public class CMSSizeGuardrail extends MinThreshold
+{
+    /**
+     * Creates a new minimum threshold guardrail.
+     *
+     * @param failThreshold   a {@link ClientState}-based provider of the value above which the operation should be aborted.
+     */
+    public CMSSizeGuardrail(ToLongFunction<ClientState> failThreshold)
+    {
+        super("minimum_cms_size",
+              null,
+              state -> -1,
+              failThreshold,
+              (isWarning, what, value, threshold) ->
+              format("The CMS size of %s is below the failure threshold of %s. " +
+                     "Reconfigure CMS so its total size (sum of replication factors across all datacenters) is at least %s.",
+                     value, threshold, threshold));
+        this.throwOnNullClientState = true;
+    }
+
+    public void guard(int totalNodes, int cmsSize)
+    {
+        // A cluster can't host more CMS replicas than it has nodes. If the request already uses every node there's
+        // nothing more to enforce (e.g. during bootstrap), and requiring more would brick reconfiguration. Otherwise
+        // the request must meet the configured minimum, even when the threshold exceeds the current cluster size, so
+        // an operator can't sidestep the floor by setting a high threshold. An operator who genuinely wants a smaller
+        // CMS should disable the guardrail. When it is disabled, guard() below is a no-op.
+        if (cmsSize >= totalNodes)
+            return;
+
+        guard(cmsSize, "CMS", false, null);
+    }
+}

src/java/org/apache/cassandra/db/guardrails/Guardrails.java

@@ -618,17 +618,7 @@ public final class Guardrails implements GuardrailsMBean
     /**
      * Guardrail on the minimum CMS size (aggregate replication factor across DCs).
      */
-    public static final MinThreshold minimumCmsSize =
-    (MinThreshold) new MinThreshold("minimum_cms_size",
-                     null,
-                     state -> -1,
-                     state -> CONFIG_PROVIDER.getOrCreate(state).getMinimumCmsSizeFailThreshold(),
-                     (isWarning, what, value, threshold) ->
-                     format("The CMS size of %s is below the failure threshold of %s. " +
-                            "Reconfigure CMS so its total size (sum of replication factors across all datacenters) is at least %s.",
-                            value, threshold, threshold))
-                     // CMS reconfiguration is a system operation with no ClientState; make it abort rather than just log.
-                     .throwOnNullClientState(true);
+    public static final CMSSizeGuardrail minimumCmsSize = new CMSSizeGuardrail(state -> CONFIG_PROVIDER.getOrCreate(state).getMinimumCmsSizeFailThreshold());
 
     /**
      * Guardrail on the maximum replication factor.

src/java/org/apache/cassandra/tcm/CMSOperations.java

@@ -188,15 +188,7 @@ public class CMSOperations implements CMSOperationsMBean
         int cmsSize = params.options.values().stream()
                                     .mapToInt(Integer::parseInt)
                                     .sum();
-        // A cluster can't host more CMS replicas than it has nodes. If the request already uses every node there's
-        // nothing more to enforce (e.g. during bootstrap), and requiring more would brick reconfiguration. Otherwise
-        // the request must meet the configured minimum, even when the threshold exceeds the current cluster size, so
-        // an operator can't sidestep the floor by setting a high threshold. An operator who genuinely wants a smaller
-        // CMS should disable the guardrail. When it is disabled, guard() below is a no-op.
-        if (cmsSize >= totalNodes)
-            return;
-
-        Guardrails.minimumCmsSize.guard(cmsSize, "CMS", false, null);
+        Guardrails.minimumCmsSize.guard(totalNodes, cmsSize);
     }
 
     @Override