fix: use SELECT COUNT(1) without VALUE in aggregate serializer test

SELECT VALUE COUNT(1) returns a scalar integer, which cannot be
deserialized as ObjectNode. The ValueUnwrapCosmosItemSerializer
extracts the _value field and tries to convert the integer to
ObjectNode, causing a deserialization failure.

Changed to SELECT COUNT(1) (without VALUE keyword) which returns
an ObjectNode like {"\": 3}, and added an assertion to verify
the count value.

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

@@ -764,18 +764,22 @@ public void queryWithAggregateAndCustomSerializer() {
             // results (e.g., COUNT) are not full documents and cannot be deserialized
             // by the envelope-wrapping serializer. The test still validates that the
             // client-level custom serializer does not leak into the internal query pipeline.
+            // NOTE: Use "SELECT COUNT(1)" (without VALUE) so the result is an ObjectNode
+            // like {"$1": 3}. "SELECT VALUE COUNT(1)" returns a scalar integer which
+            // cannot be deserialized as ObjectNode.
             CosmosQueryRequestOptions queryRequestOptions = new CosmosQueryRequestOptions()
                 .setCustomItemSerializer(CosmosItemSerializer.DEFAULT_SERIALIZER);
 
             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,
                     ObjectNode.class)
                 .stream().collect(Collectors.toList());
 
             assertThat(results).isNotNull();
             assertThat(results).hasSize(1);
+            assertThat(results.get(0).get("$1").asInt()).isEqualTo(3);
         } finally {
             for (String id : createdIds) {
                 try {