Add tests for COMPLETION logical plan optimization.

Author: afoucret

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/optimizer/LogicalPlanOptimizerTests.java

@@ -100,6 +100,7 @@
 import org.elasticsearch.xpack.esql.optimizer.rules.logical.OptimizerRules;
 import org.elasticsearch.xpack.esql.optimizer.rules.logical.PruneRedundantOrderBy;
 import org.elasticsearch.xpack.esql.optimizer.rules.logical.PushDownAndCombineLimits;
+import org.elasticsearch.xpack.esql.optimizer.rules.logical.PushDownCompletion;
 import org.elasticsearch.xpack.esql.optimizer.rules.logical.PushDownEnrich;
 import org.elasticsearch.xpack.esql.optimizer.rules.logical.PushDownEval;
 import org.elasticsearch.xpack.esql.optimizer.rules.logical.PushDownRegexExtract;
@@ -123,6 +124,7 @@
 import org.elasticsearch.xpack.esql.plan.logical.TimeSeriesAggregate;
 import org.elasticsearch.xpack.esql.plan.logical.TopN;
 import org.elasticsearch.xpack.esql.plan.logical.UnaryPlan;
+import org.elasticsearch.xpack.esql.plan.logical.inference.Completion;
 import org.elasticsearch.xpack.esql.plan.logical.join.InlineJoin;
 import org.elasticsearch.xpack.esql.plan.logical.join.Join;
 import org.elasticsearch.xpack.esql.plan.logical.join.JoinConfig;
@@ -162,6 +164,7 @@
 import static org.elasticsearch.xpack.esql.EsqlTestUtils.getFieldAttribute;
 import static org.elasticsearch.xpack.esql.EsqlTestUtils.loadMapping;
 import static org.elasticsearch.xpack.esql.EsqlTestUtils.localSource;
+import static org.elasticsearch.xpack.esql.EsqlTestUtils.randomLiteral;
 import static org.elasticsearch.xpack.esql.EsqlTestUtils.referenceAttribute;
 import static org.elasticsearch.xpack.esql.EsqlTestUtils.singleValue;
 import static org.elasticsearch.xpack.esql.EsqlTestUtils.unboundLogicalOptimizerContext;
@@ -176,6 +179,7 @@
 import static org.elasticsearch.xpack.esql.core.type.DataType.INTEGER;
 import static org.elasticsearch.xpack.esql.core.type.DataType.KEYWORD;
 import static org.elasticsearch.xpack.esql.core.type.DataType.LONG;
+import static org.elasticsearch.xpack.esql.core.type.DataType.TEXT;
 import static org.elasticsearch.xpack.esql.expression.predicate.operator.comparison.EsqlBinaryComparison.BinaryComparisonOperation.EQ;
 import static org.elasticsearch.xpack.esql.expression.predicate.operator.comparison.EsqlBinaryComparison.BinaryComparisonOperation.GT;
 import static org.elasticsearch.xpack.esql.expression.predicate.operator.comparison.EsqlBinaryComparison.BinaryComparisonOperation.GTE;
@@ -5556,7 +5560,18 @@ record PushdownShadowingGeneratingPlanTestCase(
                 )
             ),
             new PushDownEnrich()
-        ) };
+        ),
+        // | COMPLETION CONCAT(some text, x) WITH inferenceID AS y
+        new PushdownShadowingGeneratingPlanTestCase(
+            (plan, attr) -> new Completion(
+                EMPTY,
+                plan,
+                randomLiteral(TEXT),
+                new Concat(EMPTY, randomLiteral(TEXT), List.of(attr)),
+                new ReferenceAttribute(EMPTY, "y", KEYWORD)
+            ),
+            new PushDownCompletion()
+        )};
 
     /**
      * Consider