feat: include all models from provider's /v1/models

this replaces the static model listing for any provider using OpenAIMixin

test with -

 - new unit tests
 - manual for llama-api, openai, groq, gemini

```
for provider in llama-openai-compat openai groq gemini; do
   uv run llama stack build --image-type venv --providers inference=remote::provider --run &
   uv run --with llama-stack-client llama-stack-client models list | grep Total
```

results (17 sep 2025):
 - llama-api: 4
 - openai: 86
 - groq: 21
 - gemini: 66

tests/unit/providers/inference/test_openai_base_url_config.py

@@ -5,7 +5,7 @@
 # the root directory of this source tree.
 
 import os
-from unittest.mock import AsyncMock, MagicMock, patch
+from unittest.mock import MagicMock, patch
 
 from llama_stack.core.stack import replace_env_vars
 from llama_stack.providers.remote.inference.openai.config import OpenAIConfig
@@ -80,11 +80,22 @@ class TestOpenAIBaseURLConfig:
         # Mock the get_api_key method
         adapter.get_api_key = MagicMock(return_value="test-key")
 
-        # Mock the AsyncOpenAI client and its models.retrieve method
+        # Mock a model object that will be returned by models.list()
+        mock_model = MagicMock()
+        mock_model.id = "gpt-4"
+
+        # Create an async iterator that yields our mock model
+        async def mock_async_iterator():
+            yield mock_model
+
+        # Mock the AsyncOpenAI client and its models.list method
         mock_client = MagicMock()
-        mock_client.models.retrieve = AsyncMock(return_value=MagicMock())
+        mock_client.models.list = MagicMock(return_value=mock_async_iterator())
         mock_openai_class.return_value = mock_client
 
+        # Set the __provider_id__ attribute that's expected by list_models
+        adapter.__provider_id__ = "openai"
+
         # Call check_model_availability and verify it returns True
         assert await adapter.check_model_availability("gpt-4")
 
@@ -94,8 +105,8 @@ class TestOpenAIBaseURLConfig:
             base_url=custom_url,
         )
 
-        # Verify the method was called and returned True
-        mock_client.models.retrieve.assert_called_once_with("gpt-4")
+        # Verify the models.list method was called
+        mock_client.models.list.assert_called_once()
 
     @patch.dict(os.environ, {"OPENAI_BASE_URL": "https://proxy.openai.com/v1"})
     @patch("llama_stack.providers.utils.inference.openai_mixin.AsyncOpenAI")
@@ -110,11 +121,22 @@ class TestOpenAIBaseURLConfig:
         # Mock the get_api_key method
         adapter.get_api_key = MagicMock(return_value="test-key")
 
-        # Mock the AsyncOpenAI client
+        # Mock a model object that will be returned by models.list()
+        mock_model = MagicMock()
+        mock_model.id = "gpt-4"
+
+        # Create an async iterator that yields our mock model
+        async def mock_async_iterator():
+            yield mock_model
+
+        # Mock the AsyncOpenAI client and its models.list method
         mock_client = MagicMock()
-        mock_client.models.retrieve = AsyncMock(return_value=MagicMock())
+        mock_client.models.list = MagicMock(return_value=mock_async_iterator())
         mock_openai_class.return_value = mock_client
 
+        # Set the __provider_id__ attribute that's expected by list_models
+        adapter.__provider_id__ = "openai"
+
         # Call check_model_availability and verify it returns True
         assert await adapter.check_model_availability("gpt-4")