feat: Add test for Sum method in AggregateQuery

firestore/integration_test_internal/src/aggregate_query_test.cc

@@ -337,6 +337,21 @@ TEST_F(AggregateQueryTest, TestHashCode) {
             AggregateQueryHash(query1.Count()));
 }
 
+TEST_F(AggregateQueryTest, SumReturnsCorrectSum) {
+  CollectionReference collection =
+      Collection({{"a", {{"value", FieldValue::Integer(1)}}},
+                  {"b", {{"value", FieldValue::Integer(2)}}},
+                  {"c", {{"value", FieldValue::Integer(3)}}}});
+
+  AggregateQuery aggregate_query =
+      collection.Sum("value");
+  AggregateQuerySnapshot snapshot = ReadAggregate(aggregate_query);
+
+  EXPECT_EQ(6.0, snapshot.sum("value"));
+  EXPECT_EQ(aggregate_query, snapshot.query());
+}
+
+
 #if defined(__ANDROID__)
 TEST(QueryTestAndroidStub, Construction) {
   testutil::AssertWrapperConstructionContract<AggregateQuery>();

firestore/integration_test_internal/src/aggregate_sum_test.cc

@@ -0,0 +1,17 @@
+/*
+ * Copyright 2025 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// TODO: Add tests for AggregateQuery Sum