ESQL: Fix EsqlNodeSubclassTest failure from mutating

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/analysis/Analyzer.java

@@ -454,6 +454,8 @@ protected LogicalPlan rule(LogicalPlan plan, AnalyzerContext context) {
             }
             final List<Attribute> childrenOutput = new ArrayList<>();
 
+            // Gather all the children's output in case of non-unary plans; even for unaries, we need to copy because we may mutate this to
+            // simplify resolution of e.g. RENAME.
             for (LogicalPlan child : plan.children()) {
                 var output = child.output();
                 childrenOutput.addAll(output);
@@ -974,6 +976,10 @@ private LogicalPlan resolveRename(Rename rename, List<Attribute> childrenOutput)
             return new EsqlProject(rename.source(), rename.child(), projections);
         }
 
+        /**
+         * This will turn a {@link Rename} into an equivalent {@link Project}.
+         * Can mutate {@code childrenOutput}; hand this a copy if you want to avoid mutation.
+         */
         public static List<NamedExpression> projectionsForRename(Rename rename, List<Attribute> childrenOutput, Logger logger) {
             List<NamedExpression> projections = new ArrayList<>(childrenOutput);
 

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/plan/logical/Rename.java

@@ -18,6 +18,7 @@
 import org.elasticsearch.xpack.esql.core.tree.Source;
 import org.elasticsearch.xpack.esql.expression.function.UnsupportedAttribute;
 
+import java.util.ArrayList;
 import java.util.List;
 import java.util.Objects;
 
@@ -47,7 +48,11 @@ public List<Alias> renamings() {
     @Override
     public List<Attribute> output() {
         // Normally shouldn't reach here, as Rename only exists before resolution.
-        List<NamedExpression> projectionsAfterResolution = ResolveRefs.projectionsForRename(this, this.child().output(), null);
+        List<NamedExpression> projectionsAfterResolution = ResolveRefs.projectionsForRename(
+            this,
+            new ArrayList<>(this.child().output()),
+            null
+        );
 
         return Expressions.asAttributes(projectionsAfterResolution);
     }