Add unit test for ValuesExpression column-name backfill during pruning

Protects the case where leading VALUES columns stay referenced while a
later column is pruned — the path that previously repeated the pruned
column's name in newColumnNames.

Author: yvesleguennec

test/EFCore.Relational.Tests/Query/SqlTreePrunerTest.cs

@@ -0,0 +1,55 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Microsoft.EntityFrameworkCore.Query.SqlExpressions;
+
+namespace Microsoft.EntityFrameworkCore.Query;
+
+public class SqlTreePrunerTest
+{
+    [Fact]
+    public void PruneValues_preserves_leading_column_names_when_pruning_a_later_column()
+    {
+        // VALUES (_ord, Kept, Dropped) where only _ord and Kept are referenced.
+        // The first unreferenced column is at index 2, so PruneValues must backfill
+        // ColumnNames[0] and ColumnNames[1] — not repeat the pruned column's name (#38700 Copilot).
+        const string alias = "v";
+        var intMapping = IntTypeMapping.Default;
+
+        var values = new ValuesExpression(
+            alias,
+            [
+                new RowValueExpression(
+                [
+                    new SqlConstantExpression(0, intMapping),
+                    new SqlConstantExpression(10, intMapping),
+                    new SqlConstantExpression(20, intMapping)
+                ])
+            ],
+            [
+                RelationalQueryableMethodTranslatingExpressionVisitor.ValuesOrderingColumnName,
+                "Kept",
+                "Dropped"
+            ]);
+
+        var pruner = new TestSqlTreePruner();
+        pruner.RegisterColumn(alias, RelationalQueryableMethodTranslatingExpressionVisitor.ValuesOrderingColumnName);
+        pruner.RegisterColumn(alias, "Kept");
+
+        var pruned = pruner.PruneValuesPublic(values);
+
+        Assert.Equal(
+            [RelationalQueryableMethodTranslatingExpressionVisitor.ValuesOrderingColumnName, "Kept"],
+            pruned.ColumnNames);
+        Assert.Equal(2, Assert.Single(pruned.RowValues!).Values.Count);
+    }
+
+    private sealed class TestSqlTreePruner : SqlTreePruner
+    {
+        public void RegisterColumn(string tableAlias, string columnName)
+            => Visit(new ColumnExpression(columnName, tableAlias, typeof(int), IntTypeMapping.Default, nullable: false));
+
+        public ValuesExpression PruneValuesPublic(ValuesExpression values)
+            => PruneValues(values);
+    }
+}