ES|QL: Make ResolveUnionTypes rule stateless (elastic#136492)

Author: luigidellaquila

docs/changelog/136492.yaml

@@ -0,0 +1,5 @@
+pr: 136492
+summary: Make `ResolveUnionTypes` rule stateless
+area: ES|QL
+type: bug
+issues: []

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

@@ -1845,15 +1845,13 @@ private static class ResolveUnionTypes extends Rule<LogicalPlan, LogicalPlan> {
 
         record TypeResolutionKey(String fieldName, DataType fieldType) {}
 
-        private List<FieldAttribute> unionFieldAttributes;
-
         @Override
         public LogicalPlan apply(LogicalPlan plan) {
-            unionFieldAttributes = new ArrayList<>();
-            return plan.transformUp(LogicalPlan.class, p -> p.childrenResolved() == false ? p : doRule(p));
+            List<FieldAttribute> unionFieldAttributes = new ArrayList<>();
+            return plan.transformUp(LogicalPlan.class, p -> p.childrenResolved() == false ? p : doRule(p, unionFieldAttributes));
         }
 
-        private LogicalPlan doRule(LogicalPlan plan) {
+        private LogicalPlan doRule(LogicalPlan plan, List<FieldAttribute> unionFieldAttributes) {
             Holder<Integer> alreadyAddedUnionFieldAttributes = new Holder<>(unionFieldAttributes.size());
             // Collect field attributes from previous runs
             if (plan instanceof EsRelation rel) {