Support to access model gardens using endpoints: rawPredict or streamRawPredict

## Description

The Vertex AI documentation describes that we can select one of the different models: open, partner, gemini-family and access them using [curl](https://docs.cloud.google.com/vertex-ai/generative-ai/docs/partner-models/claude/use-claude#use_a_curl_command), python and probably this Java SDK (as documented [here](https://docs.cloud.google.com/vertex-ai/generative-ai/docs/start/libraries)) but when we use the following code

```java
Client client = null;
try {
GoogleCredentials creds = GoogleCredentials.getApplicationDefault();
creds.refreshIfExpired();

client = Client.builder()
.credentials(creds)
.build();
} catch (Exception e) {
throw new RuntimeException(e);
}

GenerateContentResponse response =
client.models.generateContent("anthropic/claude-opus-4-6", "What is your name?", null);
```
then we got this error
```
com.google.genai.errors.ClientException: 400 . claude-opus-4-6 is not supported in the generateContent API.
at com.google.genai.errors.ApiException.throwFromResponse (ApiException.java:94)
at com.google.genai.HttpApiResponse.getBody (HttpApiResponse.java:37)
at com.google.genai.Models.processResponseForPrivateGenerateContent (Models.java:5480)
at com.google.genai.Models.privateGenerateContent (Models.java:5546)
at com.google.genai.Models.generateContent (Models.java:7076)
at com.google.genai.Models.generateContent (Models.java:7150)
at com.google.genai.examples.GenerateContent.main (GenerateContent.java:89)
```

## Suggestion

This Java SDK project is a great library as it allows third-party projects to access the Google Cloud platform and Vertex AI using HTTP(s) with JSON body messages instead of using gRPC.

As the access to one of the model partners like Claude, Mistral, etc relies on the following endpoints
```
https://LOCATION-aiplatform.googleapis.com/v1/projects/PROJECT_ID/locations/LOCATION/publishers/anthropic/models/MODEL:rawPredict
https://LOCATION-aiplatform.googleapis.com/v1/projects/PROJECT_ID/locations/LOCATION/publishers/anthropic/models/MODEL:streamRawPredict
```
I suggest that this Java SDK also support them. Here is the API documentation: https://docs.cloud.google.com/vertex-ai/generative-ai/docs/reference/rest/v1/projects.locations.endpoints/rawPredict

FYI: The projects [Langchain4j](https://github.com/langchain4j/langchain4j/tree/main/langchain4j-vertex-ai) and [Quarkus Langchains4j](https://github.com/quarkiverse/quarkus-langchain4j/tree/main/model-providers/google/vertex-ai) currently use the gRPC code of the following GAV and will be more than happy to use now this SDK with HTTP(S).
```

com.google.cloud
google-cloud-aiplatform
3.69.0

```