fix: use SELECT COUNT(1) instead of SELECT VALUE COUNT(1) in aggregate serializer test

Custom serializers' deserialize(Map<String, Object>, Class<T>) API is designed
for object-to-POJO mapping. SELECT VALUE COUNT(1) returns a scalar that gets
wrapped in a {"_value": N} Document by the aggregate pipeline, which cannot
be deserialized as Integer.class through a custom serializer.

Changed to SELECT COUNT(1) which returns an object {"$1": 3} that properly
goes through the Map-based deserialization path.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: xinlian12

sdk/cosmos/azure-cosmos-tests/src/test/java/com/azure/cosmos/CosmosItemSerializerTest.java

@@ -771,17 +771,21 @@ public void queryWithAggregateAndCustomSerializer() {
             CosmosQueryRequestOptions queryRequestOptions = new CosmosQueryRequestOptions()
                 .setCustomItemSerializer(clientSerializer);
 
-            // SELECT VALUE COUNT(1) returns a scalar integer, so use Integer.class.
-            List<Integer> results = container
+            // SELECT COUNT(1) returns an object (e.g. {"$1": 3}) rather than a scalar.
+            // Custom serializers work with Map<String, Object> which is object-level
+            // deserialization - they cannot handle scalar VALUE queries like
+            // SELECT VALUE COUNT(1) because the aggregate pipeline wraps the scalar
+            // in a {"_value": N} Document that can't be deserialized as Integer.
+            List<ObjectNode> results = container
                 .queryItems(
-                    "SELECT VALUE COUNT(1) FROM c WHERE c.mypk = '" + pkValue + "'",
+                    "SELECT COUNT(1) FROM c WHERE c.mypk = '" + pkValue + "'",
                     queryRequestOptions,
-                    Integer.class)
+                    ObjectNode.class)
                 .stream().collect(Collectors.toList());
 
             assertThat(results).isNotNull();
             assertThat(results).hasSize(1);
-            assertThat(results.get(0)).isEqualTo(3);
+            assertThat(results.get(0).get("$1").asInt()).isEqualTo(3);
         } finally {
             for (String id : createdIds) {
                 try {