[ML] Fix serialization order for model_size_stats (#66015)

Due to a backporting mistake, master and 7.x were
serializing the members of model_size_stats in
different orders.  This led to BWC test failures
on master.

This change makes the ordering in 7.x the same as
the ordering in master.  It will be tested when
the mute done in #65895 is reverted on the master
branch.

Fixes #65893

Author: David Roberts

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ml/job/process/autodetect/state/ModelSizeStats.java

@@ -226,6 +226,15 @@ public ModelSizeStats(StreamInput in) throws IOException {
         totalPartitionFieldCount = in.readVLong();
         bucketAllocationFailuresCount = in.readVLong();
         memoryStatus = MemoryStatus.readFromStream(in);
+        if (in.getVersion().onOrAfter(Version.V_7_11_0)) {
+            if (in.readBoolean()) {
+                assignmentMemoryBasis = AssignmentMemoryBasis.readFromStream(in);
+            } else {
+                assignmentMemoryBasis = null;
+            }
+        } else {
+            assignmentMemoryBasis = null;
+        }
         if (in.getVersion().onOrAfter(Version.V_7_7_0)) {
             categorizedDocCount = in.readVLong();
             totalCategoryCount = in.readVLong();
@@ -247,15 +256,6 @@ public ModelSizeStats(StreamInput in) throws IOException {
             failedCategoryCount = 0;
             categorizationStatus = CategorizationStatus.OK;
         }
-        if (in.getVersion().onOrAfter(Version.V_7_11_0)) {
-            if (in.readBoolean()) {
-                assignmentMemoryBasis = AssignmentMemoryBasis.readFromStream(in);
-            } else {
-                assignmentMemoryBasis = null;
-            }
-        } else {
-            assignmentMemoryBasis = null;
-        }
         logTime = new Date(in.readVLong());
         timestamp = in.readBoolean() ? new Date(in.readVLong()) : null;
     }
@@ -286,6 +286,14 @@ public void writeTo(StreamOutput out) throws IOException {
         out.writeVLong(totalPartitionFieldCount);
         out.writeVLong(bucketAllocationFailuresCount);
         memoryStatus.writeTo(out);
+        if (out.getVersion().onOrAfter(Version.V_7_11_0)) {
+            if (assignmentMemoryBasis != null) {
+                out.writeBoolean(true);
+                assignmentMemoryBasis.writeTo(out);
+            } else {
+                out.writeBoolean(false);
+            }
+        }
         if (out.getVersion().onOrAfter(Version.V_7_7_0)) {
             out.writeVLong(categorizedDocCount);
             out.writeVLong(totalCategoryCount);
@@ -297,14 +305,6 @@ public void writeTo(StreamOutput out) throws IOException {
             }
             categorizationStatus.writeTo(out);
         }
-        if (out.getVersion().onOrAfter(Version.V_7_11_0)) {
-            if (assignmentMemoryBasis != null) {
-                out.writeBoolean(true);
-                assignmentMemoryBasis.writeTo(out);
-            } else {
-                out.writeBoolean(false);
-            }
-        }
         out.writeVLong(logTime.getTime());
         boolean hasTimestamp = timestamp != null;
         out.writeBoolean(hasTimestamp);