fix: update hard-coded google model names (#4212)

When we send the model names to Google's openai API, we must use the
"google" name prefix. Google does not recognize the "vertexai" model
names.

Closes #4211

```bash
uv venv --python python312
. .venv/bin/activate
llama stack list-deps starter | xargs -L1 uv pip install
llama stack run starter
```

Test that this shows the gemini models with their correct names:
```bash
curl http://127.0.0.1:8321/v1/models | jq '.data | map(select(.custom_metadata.provider_id == "vertexai"))'
```

Test that this chat completion works:
```bash
curl -X POST   -H "Content-Type: application/json"   "http://127.0.0.1:8321/v1/chat/completions"   -d '{
        "model": "vertexai/google/gemini-2.5-flash",
        "messages": [
          {
            "role": "system",
            "content": "You are a helpful assistant."
          },
          {
            "role": "user",
            "content": "Hello! Can you tell me a joke?"
          }
        ],
        "temperature": 1.0,
        "max_tokens": 256
      }'
```

(cherry picked from commit dabebdd230)
Signed-off-by: Charlie Doern <cdoern@redhat.com>

llama_stack/providers/remote/inference/vertexai/vertexai.py

@@ -4,6 +4,7 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
+from collections.abc import Iterable
 
 import google.auth.transport.requests
 from google.auth import default
@@ -42,3 +43,12 @@ class VertexAIInferenceAdapter(OpenAIMixin):
         Source: https://cloud.google.com/vertex-ai/generative-ai/docs/start/openai
         """
         return f"https://{self.config.location}-aiplatform.googleapis.com/v1/projects/{self.config.project}/locations/{self.config.location}/endpoints/openapi"
+
+    async def list_provider_model_ids(self) -> Iterable[str]:
+        """
+        VertexAI doesn't currently offer a way to query a list of available models from Google's Model Garden
+        For now we return a hardcoded version of the available models
+
+        :return: An iterable of model IDs
+        """
+        return ["google/gemini-2.0-flash", "google/gemini-2.5-flash", "google/gemini-2.5-pro"]