Add VectorSearchQueryBuilder unit tests

Verifies knn query is placed in scoring (must) context, not wrapped
in bool.filter when no WHERE clause is present.

Signed-off-by: Eric Wei <mengwei.eric@gmail.com>

Author: mengweieric

opensearch/src/test/java/org/opensearch/sql/opensearch/storage/scan/VectorSearchQueryBuilderTest.java

@@ -0,0 +1,50 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package org.opensearch.sql.opensearch.storage.scan;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.mockito.Mockito.mock;
+
+import org.junit.jupiter.api.Test;
+import org.opensearch.index.query.QueryBuilder;
+import org.opensearch.index.query.WrapperQueryBuilder;
+import org.opensearch.sql.common.setting.Settings;
+import org.opensearch.sql.opensearch.data.value.OpenSearchExprValueFactory;
+import org.opensearch.sql.opensearch.request.OpenSearchRequestBuilder;
+
+class VectorSearchQueryBuilderTest {
+
+  @Test
+  void knnQuerySetAsScoringQuery() {
+    var requestBuilder = createRequestBuilder();
+    var knnQuery = new WrapperQueryBuilder("eyJrbm4iOnt9fQ==");
+
+    new VectorSearchQueryBuilder(requestBuilder, knnQuery);
+
+    QueryBuilder query = requestBuilder.getSourceBuilder().query();
+    assertTrue(
+        query instanceof WrapperQueryBuilder,
+        "knn query should be set directly as top-level query (scoring context)");
+  }
+
+  @Test
+  void knnQueryNotWrappedInFilterWhenNoWhere() {
+    var requestBuilder = createRequestBuilder();
+    var knnQuery = new WrapperQueryBuilder("eyJrbm4iOnt9fQ==");
+
+    new VectorSearchQueryBuilder(requestBuilder, knnQuery);
+
+    QueryBuilder query = requestBuilder.getSourceBuilder().query();
+    assertTrue(
+        query instanceof WrapperQueryBuilder,
+        "Without WHERE clause, knn query should NOT be wrapped in bool.filter");
+  }
+
+  private OpenSearchRequestBuilder createRequestBuilder() {
+    return new OpenSearchRequestBuilder(
+        mock(OpenSearchExprValueFactory.class), 10000, mock(Settings.class));
+  }
+}