Combine cluster and project tasks in xcontent for single project (elastic#124613)

This PR combines both cluster and project tasks under persistent_tasks
for XContent output of Metadata when it contains only a single project,
i.e. there will be no cluster_persistent_tasks in such output.  This is
to maintain the existing output format when the cluster is not
multi-project enabled.

Relates: MP-1945

Author: ywangd

server/src/main/java/org/elasticsearch/cluster/metadata/Metadata.java

@@ -744,7 +744,26 @@ public Iterator<? extends ToXContent> toXContentChunked(ToXContent.Params p) {
             // and not include it in the project xcontent output (through the lack of multi-project params)
             clusterReservedState.putAll(project.reservedStateMetadata());
 
-            @FixForMultiProject(description = "consider include cluster-scoped persistent tasks")
+            // Similarly, combine cluster and project persistent tasks and report them under a single key
+            Iterator<ToXContent> customs = Iterators.flatMap(customs().entrySet().iterator(), entry -> {
+                if (entry.getValue().context().contains(context)
+                    && ClusterPersistentTasksCustomMetadata.TYPE.equals(entry.getKey()) == false) {
+                    return ChunkedToXContentHelper.object(entry.getKey(), entry.getValue().toXContentChunked(p));
+                } else {
+                    return Collections.emptyIterator();
+                }
+            });
+            final var combinedTasks = PersistentTasksCustomMetadata.combine(
+                ClusterPersistentTasksCustomMetadata.get(this),
+                PersistentTasksCustomMetadata.get(project)
+            );
+            if (combinedTasks != null) {
+                customs = Iterators.concat(
+                    customs,
+                    ChunkedToXContentHelper.object(PersistentTasksCustomMetadata.TYPE, combinedTasks.toXContentChunked(p))
+                );
+            }
+
             final var iterators = Iterators.concat(start, Iterators.single((builder, params) -> {
                 builder.field("cluster_uuid", clusterUUID);
                 builder.field("cluster_uuid_committed", clusterUUIDCommitted);
@@ -754,12 +773,7 @@ public Iterator<? extends ToXContent> toXContentChunked(ToXContent.Params p) {
             }),
                 persistentSettings,
                 project.toXContentChunked(p),
-                Iterators.flatMap(
-                    customs.entrySet().iterator(),
-                    entry -> entry.getValue().context().contains(context)
-                        ? ChunkedToXContentHelper.object(entry.getKey(), entry.getValue().toXContentChunked(p))
-                        : Collections.emptyIterator()
-                ),
+                customs,
                 ChunkedToXContentHelper.object("reserved_state", clusterReservedState.values().iterator()),
                 ChunkedToXContentHelper.endObject()
             );

server/src/main/java/org/elasticsearch/cluster/metadata/ProjectMetadata.java

@@ -37,6 +37,7 @@
 import org.elasticsearch.index.IndexNotFoundException;
 import org.elasticsearch.index.IndexSettings;
 import org.elasticsearch.index.IndexVersion;
+import org.elasticsearch.persistent.PersistentTasksCustomMetadata;
 import org.elasticsearch.plugins.FieldPredicate;
 import org.elasticsearch.plugins.MapperPlugin;
 import org.elasticsearch.transport.Transports;
@@ -2111,14 +2112,18 @@ public Iterator<? extends ToXContent> toXContentChunked(ToXContent.Params p) {
             ? ChunkedToXContentHelper.object("indices", indices().values().iterator())
             : Collections.emptyIterator();
 
-        Iterator<ToXContent> customs = Iterators.flatMap(
-            customs().entrySet().iterator(),
-            entry -> entry.getValue().context().contains(context)
-                ? ChunkedToXContentHelper.object(entry.getKey(), entry.getValue().toXContentChunked(p))
-                : Collections.emptyIterator()
-        );
-
         final var multiProject = p.paramAsBoolean("multi-project", false);
+        Iterator<ToXContent> customs = Iterators.flatMap(customs().entrySet().iterator(), entry -> {
+            if (entry.getValue().context().contains(context)
+                // Include persistent tasks in the output only when multi-project=true.
+                // In single-project-mode (multi-project=false), we already output them in Metadata.
+                && (multiProject || PersistentTasksCustomMetadata.TYPE.equals(entry.getKey()) == false)) {
+                return ChunkedToXContentHelper.object(entry.getKey(), entry.getValue().toXContentChunked(p));
+            } else {
+                return Collections.emptyIterator();
+            }
+        });
+
         return Iterators.concat(
             ChunkedToXContentHelper.object(
                 "templates",