feat(internal): add image_url download feature to OpenAIMixin

simplify Ollama inference adapter by -
 - moving image_url download code to OpenAIMixin
 - being a ModelRegistryHelper instead of having one (mypy blocks check_model_availability method assignment)

testing -
 - add unit tests for new download feature
 - add integration tests for openai_chat_completion w/ image_url (close test gap)

tests/unit/providers/utils/inference/test_openai_mixin.py

@@ -4,11 +4,11 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
-from unittest.mock import MagicMock, PropertyMock, patch
+from unittest.mock import AsyncMock, MagicMock, PropertyMock, patch
 
 import pytest
 
-from llama_stack.apis.inference import Model
+from llama_stack.apis.inference import Model, OpenAIUserMessageParam
 from llama_stack.apis.models import ModelType
 from llama_stack.providers.utils.inference.openai_mixin import OpenAIMixin
 
@@ -27,8 +27,17 @@ class OpenAIMixinImpl(OpenAIMixin):
 
 @pytest.fixture
 def mixin():
-    """Create a test instance of OpenAIMixin"""
-    return OpenAIMixinImpl()
+    """Create a test instance of OpenAIMixin with mocked model_store"""
+    mixin_instance = OpenAIMixinImpl()
+
+    # just enough to satisfy _get_provider_model_id calls
+    mock_model_store = MagicMock()
+    mock_model = MagicMock()
+    mock_model.provider_resource_id = "test-provider-resource-id"
+    mock_model_store.get_model = AsyncMock(return_value=mock_model)
+    mixin_instance.model_store = mock_model_store
+
+    return mixin_instance
 
 
 @pytest.fixture
@@ -181,3 +190,71 @@ class TestOpenAIMixinCacheBehavior:
             assert "some-mock-model-id" in mixin._model_cache
             assert "another-mock-model-id" in mixin._model_cache
             assert "final-mock-model-id" in mixin._model_cache
+
+
+class TestOpenAIMixinImagePreprocessing:
+    """Test cases for image preprocessing functionality"""
+
+    async def test_openai_chat_completion_with_image_preprocessing_enabled(self, mixin):
+        """Test that image URLs are converted to base64 when download_images is True"""
+        mixin.download_images = True
+
+        message = OpenAIUserMessageParam(
+            role="user",
+            content=[
+                {"type": "text", "text": "What's in this image?"},
+                {"type": "image_url", "image_url": {"url": "http://example.com/image.jpg"}},
+            ],
+        )
+
+        mock_client = MagicMock()
+        mock_response = MagicMock()
+        mock_client.chat.completions.create = AsyncMock(return_value=mock_response)
+
+        with patch.object(type(mixin), "client", new_callable=PropertyMock, return_value=mock_client):
+            with patch("llama_stack.providers.utils.inference.openai_mixin.localize_image_content") as mock_localize:
+                mock_localize.return_value = (b"fake_image_data", "jpeg")
+
+                await mixin.openai_chat_completion(model="test-model", messages=[message])
+
+            mock_localize.assert_called_once_with("http://example.com/image.jpg")
+
+            mock_client.chat.completions.create.assert_called_once()
+            call_args = mock_client.chat.completions.create.call_args
+            processed_messages = call_args[1]["messages"]
+            assert len(processed_messages) == 1
+            content = processed_messages[0]["content"]
+            assert len(content) == 2
+            assert content[0]["type"] == "text"
+            assert content[1]["type"] == "image_url"
+            assert content[1]["image_url"]["url"] == "data:image/jpeg;base64,ZmFrZV9pbWFnZV9kYXRh"
+
+    async def test_openai_chat_completion_with_image_preprocessing_disabled(self, mixin):
+        """Test that image URLs are not modified when download_images is False"""
+        mixin.download_images = False  # explicitly set to False
+
+        message = OpenAIUserMessageParam(
+            role="user",
+            content=[
+                {"type": "text", "text": "What's in this image?"},
+                {"type": "image_url", "image_url": {"url": "http://example.com/image.jpg"}},
+            ],
+        )
+
+        mock_client = MagicMock()
+        mock_response = MagicMock()
+        mock_client.chat.completions.create = AsyncMock(return_value=mock_response)
+
+        with patch.object(type(mixin), "client", new_callable=PropertyMock, return_value=mock_client):
+            with patch("llama_stack.providers.utils.inference.openai_mixin.localize_image_content") as mock_localize:
+                await mixin.openai_chat_completion(model="test-model", messages=[message])
+
+            mock_localize.assert_not_called()
+
+            mock_client.chat.completions.create.assert_called_once()
+            call_args = mock_client.chat.completions.create.call_args
+            processed_messages = call_args[1]["messages"]
+            assert len(processed_messages) == 1
+            content = processed_messages[0]["content"]
+            assert len(content) == 2
+            assert content[1]["image_url"]["url"] == "http://example.com/image.jpg"