fix: return to hardcoded model IDs for Vertex AI (#4041)

# What does this PR do?
partial revert of b67aef2

Vertex AI doesn't offer an endpoint for listing models from Google's
Model Garden

Return to hardcoded values until such an endpoint is available

Closes #3988 

## Test Plan
Server side, set up your Vertex AI env vars (`VERTEX_AI_PROJECT`,
`VERTEX_AI_LOCATION`, and `GOOGLE_APPLICATION_CREDENTIALS`) and run the
starter distribution
```bash
$ llama stack list-deps starter | xargs -L1 uv pip install
$ llama stack run starter
```

Client side, formerly broken cURL requests now working
```bash
$ curl http://127.0.0.1:8321/v1/models | jq '.data | map(select(.provider_id == "vertexai"))'
[
  {
    "identifier": "vertexai/vertex_ai/gemini-2.0-flash",
    "provider_resource_id": "vertex_ai/gemini-2.0-flash",
    "provider_id": "vertexai",
    "type": "model",
    "metadata": {},
    "model_type": "llm"
  },
  {
    "identifier": "vertexai/vertex_ai/gemini-2.5-flash",
    "provider_resource_id": "vertex_ai/gemini-2.5-flash",
    "provider_id": "vertexai",
    "type": "model",
    "metadata": {},
    "model_type": "llm"
  },
  {
    "identifier": "vertexai/vertex_ai/gemini-2.5-pro",
    "provider_resource_id": "vertex_ai/gemini-2.5-pro",
    "provider_id": "vertexai",
    "type": "model",
    "metadata": {},
    "model_type": "llm"
  }
]
$ curl -fsS http://127.0.0.1:8321/v1/openai/v1/chat/completions -H "Content-Type: application/json" -d "{\"model\": \"vertexai/vertex_a
i/gemini-2.5-flash\", \"messages\": [{\"role\": \"user\", \"content\": \"Hello\"}], \"max_tokens\": 128, \"temperature\": 0.0}" | jq 
{                                                                                                                                    
  "id": "p8oIaYiQF8_PptQPo-GH8QQ",                                                                                                   
  "choices": [                                                                                                                       
    {                                                                                                                                
      "finish_reason": "stop",                                                                                                       
      "index": 0,                                                                                                                    
      "logprobs": null,                                                                                                              
      "message": {                                                                                                                   
        "content": "Hello there! How can I help you today?",                                                                         
        "refusal": null,                                                                                                             
        "role": "assistant",                                                                                                         
        "annotations": null,                                                                                                         
        "audio": null,                                                                                                               
        "function_call": null,
        "tool_calls": null
      }
    }
  ],
...
```

Signed-off-by: Nathan Weinberg <[email protected]>

Author: nathan-weinberg

src/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 @@ def get_base_url(self) -> str:
         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 ["vertexai/gemini-2.0-flash", "vertexai/gemini-2.5-flash", "vertexai/gemini-2.5-pro"]