Support Custom Endpoint from Vertex AI Model Garden

I am trying to build a sample app that use Gemma3 custom model deployed on Vertex AI Model Garden.
The SDK seems does not provide a way to define custom endpoints like :
https://{endpointId}.{location}-{projectId}.prediction.vertexai.goog/v1/projects/{projectId}/locations/{location}/models/endpoints/{endpointId}:generateContent

When I try to use endpoint , the SDK will always include publishers/google in the URL which cause the app to fail with 404 UNIMPLEMENTED error.
wrong URL : https://{endpointId}.{location}-{projectId}.prediction.vertexai.goog/v1/projects/{projectId}/locations/{location}/**publishers/google/**models/endpoints/{endpointId}:generateContent

here is my code.
same issue occurs when using PredictionServiceClient.
`String endpoint = String.format("%s.%s-%s.prediction.vertexai.goog:443", endpointId, location,projectId);

VertexAI vertexAi = new VertexAI.Builder()
.setProjectId(projectId)
.setLocation(location)
.setTransport(Transport.REST)
.setApiEndpoint(endpoint)
.build();

GenerativeModel model = new GenerativeModel("endpoints/7871795169387872256", vertexAi);`

首先以报告中的端点格式作为复现案例，跟踪 VertexAI.Builder、GenerativeModel 和 PredictionServiceClient 对自定义端点的处理。完成的标准是：可以使用自定义的 Vertex AI Model Garden 端点，而 SDK 不再添加 publishers/google，且请求不再返回 404 UNIMPLEMENTED。

由索引模型根据 Issue 内容生成。