[v1-google-cloud-discoveryengine]: Still receiving as gRpc struct even use HttpJsonTransportProvider

First of all, I'm very new with this service google-cloud-discoveryengine.
I trying to call https://discoveryengine.googleapis.com/v1 for vertex search with client lib from google cloud and i want the result as json, btw i always receive struct which is not json response. So i go to read this documentation https://cloud.google.com/java/docs/reference/google-cloud-discoveryengine/latest/com.google.cloud.discoveryengine.v1.SearchServiceClient, they said that use
SearchServiceSettings searchServiceSettings = SearchServiceSettings.newHttpJsonBuilder().build();
but i still get struct response.

can u help me solve this problem? or tell me what i did wrong
this is my code

SearchServiceSettings searchServiceSettings = SearchServiceSettings.newHttpJsonBuilder().build();
try (SearchServiceClient searchServiceClient = SearchServiceClient.create(searchServiceSettings)) {
SearchRequest request =
SearchRequest.newBuilder()
.setServingConfig(
ServingConfigName.formatProjectLocationCollectionDataStoreServingConfigName(
projectId, location, collectionId, dataStoreId, servingConfigId))
.setSpellCorrectionSpec(
SearchRequest.SpellCorrectionSpec.newBuilder()
.setMode(SearchRequest.SpellCorrectionSpec.Mode.AUTO).build()
)
.setQueryExpansionSpec(
SearchRequest.QueryExpansionSpec.newBuilder()
.setCondition(SearchRequest.QueryExpansionSpec.Condition.AUTO).build()
)
.setQuery(searchQuery)
.setPageSize(1)
.build();
SearchServiceClient.SearchPagedResponse a = searchServiceClient.search(request);
SearchResponse response = a.getPage().getResponse();
List results = response.getResultsList();
SearchResponse.SearchResult res = results.getFirst();

this is some part of my response
```
results {
id: "1250"
document {
name: "projects/272270411648/locations/global/collections/default_collection/dataStores/mordee-doctor-info-test/branches/0/documents/1250"
id: "1250"
struct_data {
fields {
key: "availableChannels"
value {
list_value {
values {
string_value: "Video"
}
values {
string_value: "Voice"
}
}
}
}
fields {
key: "availableLanguages"
value {
list_value {
values {
string_value: "th-TH"
}
}
}
}
fields {
key: "certificate"
value {
struct_value {
fields {
key: "en"
value {
string_value: "Bachelor of Science Program in Food Technology and Nutrition, Mahasarakham University "
}
}
fields {
key: "th"
value {
string_value: "Master of Science Program in Food Technology, Khonkaen University "
}
}
}
}
}
```

note* i try v1, v1beta, and v1alpha already

Java Version: java 22.0.1
Quarkus Version: 3.11.1

Thanks!