Fix explain unit test to verify actual logical plan content

Use a real Calcite LogicalValues node instead of a mock so
RelOptUtil.toString() produces a deterministic plan. Assert the exact
expected logical plan string instead of just checking non-null.

Signed-off-by: Kai Huang <kaihuang@amazon.com>
Signed-off-by: Kai Huang <ahkcs@amazon.com>

Author: ahkcs

core/src/test/java/org/opensearch/sql/executor/analytics/AnalyticsExecutionEngineTest.java

@@ -7,6 +7,7 @@
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.when;
@@ -249,43 +250,29 @@ void physicalPlanExplain_callsOnFailure() {
 
   @Test
   void explainRelNode_returnsLogicalPlan() {
-    RelNode relNode = mockRelNode("name", SqlTypeName.VARCHAR, "age", SqlTypeName.INTEGER);
+    // Build a real RelNode so RelOptUtil.toString() produces a deterministic plan
+    org.apache.calcite.tools.FrameworkConfig config =
+        org.apache.calcite.tools.Frameworks.newConfigBuilder().build();
+    org.apache.calcite.tools.RelBuilder builder =
+        org.apache.calcite.tools.RelBuilder.create(config);
+    RelNode relNode = builder.values(new String[] {"name", "age"}, "Alice", 30, "Bob", 25).build();
 
     AtomicReference<ExplainResponse> ref = new AtomicReference<>();
-    AtomicReference<Exception> errorRef = new AtomicReference<>();
     engine.explain(
         relNode,
         org.opensearch.sql.ast.statement.ExplainMode.STANDARD,
         mockContext,
-        new ResponseListener<ExplainResponse>() {
-          @Override
-          public void onResponse(ExplainResponse response) {
-            ref.set(response);
-          }
-
-          @Override
-          public void onFailure(Exception e) {
-            errorRef.set(e);
-          }
-        });
-
-    // RelOptUtil.toString on a mock may throw or succeed depending on mock setup.
-    // Accept either a successful response or a caught failure.
-    assertTrue(
-        ref.get() != null || errorRef.get() != null,
-        "Either onResponse or onFailure should have been called");
-    if (ref.get() != null) {
-      assertNotNull(ref.get().getCalcite(), "Explain should have calcite field");
-      assertNotNull(ref.get().getCalcite().getLogical(), "Explain should have logical plan");
-      System.out.println(
-          "\n--- explainRelNode_returnsLogicalPlan ---\nLogical: "
-              + ref.get().getCalcite().getLogical()
-              + "Physical: "
-              + ref.get().getCalcite().getPhysical()
-              + "\nExtended: "
-              + ref.get().getCalcite().getExtended()
-              + "\n--- End ---");
-    }
+        captureExplainListener(ref));
+
+    assertNotNull(ref.get(), "ExplainResponse should not be null");
+    String expectedLogical = "LogicalValues(tuples=[[{ 'Alice', 30 }, { 'Bob', 25 }]])\n";
+    assertEquals(expectedLogical, ref.get().getCalcite().getLogical(), "Logical plan mismatch");
+    assertNull(ref.get().getCalcite().getPhysical(), "Physical plan should be null");
+    assertNull(ref.get().getCalcite().getExtended(), "Extended plan should be null");
+    System.out.println(
+        "\n--- explainRelNode_returnsLogicalPlan ---\n"
+            + ref.get().getCalcite().getLogical()
+            + "--- End ---");
   }
 
   @Test
@@ -325,6 +312,21 @@ private QueryResponse executeAndCapture(RelNode relNode) {
     return ref.get();
   }
 
+  private ResponseListener<ExplainResponse> captureExplainListener(
+      AtomicReference<ExplainResponse> ref) {
+    return new ResponseListener<ExplainResponse>() {
+      @Override
+      public void onResponse(ExplainResponse response) {
+        ref.set(response);
+      }
+
+      @Override
+      public void onFailure(Exception e) {
+        throw new AssertionError("Unexpected failure", e);
+      }
+    };
+  }
+
   private Exception executeAndCaptureError(RelNode relNode) {
     AtomicReference<Exception> ref = new AtomicReference<>();
     engine.execute(