Expose build flavor again in nodes info (#98021)

The nodes info returns some information from the build. However, the
flavor is still hardcoded to default, even though flavor was added back
to Build. This commit exposes the build flavor again in the nodes
info response. It also fixes the build extension to be accessible in
serverless.

server/src/main/java/module-info.java

@@ -277,7 +277,7 @@ module org.elasticsearch.server {
     exports org.elasticsearch.indices.recovery.plan;
     exports org.elasticsearch.indices.store;
     exports org.elasticsearch.ingest;
-    exports org.elasticsearch.internal to org.elasticsearch.serverless.version;
+    exports org.elasticsearch.internal to org.elasticsearch.serverless.version, org.elasticsearch.serverless.buildinfo;
     exports org.elasticsearch.lucene.analysis.miscellaneous;
     exports org.elasticsearch.lucene.grouping;
     exports org.elasticsearch.lucene.queries;

server/src/main/java/org/elasticsearch/action/admin/cluster/node/info/NodesInfoResponse.java

@@ -65,8 +65,7 @@ public class NodesInfoResponse extends BaseNodesResponse<NodeInfo> implements To
 
             builder.field("version", nodeInfo.getVersion());
             builder.field("transport_version", nodeInfo.getTransportVersion().id());
-            // flavor no longer exists, but we keep it here for backcompat
-            builder.field("build_flavor", "default");
+            builder.field("build_flavor", nodeInfo.getBuild().flavor());
             builder.field("build_type", nodeInfo.getBuild().type().displayName());
             builder.field("build_hash", nodeInfo.getBuild().hash());
             if (nodeInfo.getTotalIndexingBuffer() != null) {