fix vllm base64 image inference (#815)

# What does this PR do?

- fix base64 based image url for vllm
- add a test case for base64 based image_url
- fixes issue: https://github.com/meta-llama/llama-stack/issues/571

## Test Plan

```
LLAMA_STACK_BASE_URL=http://localhost:8321 pytest -v ./tests/client-sdk/inference/test_inference.py::test_image_chat_completion_base64_url
```

<img width="991" alt="image"
src="https://github.com/user-attachments/assets/d56381ba-6777-4d23-9da9-81f73ce93566"
/>

## Sources

Please link relevant resources if necessary.


## Before submitting

- [ ] This PR fixes a typo or improves the docs (you can dismiss the
other checks if that's the case).
- [ ] Ran pre-commit to handle lint / formatting issues.
- [ ] Read the [contributor
guideline](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [ ] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

llama_stack/providers/remote/inference/vllm/vllm.py

@@ -176,10 +176,8 @@ class VLLMInferenceAdapter(Inference, ModelsProtocolPrivate):
         media_present = request_has_media(request)
         if isinstance(request, ChatCompletionRequest):
             if media_present:
-                # vllm does not seem to work well with image urls, so we download the images
                 input_dict["messages"] = [
-                    await convert_message_to_openai_dict(m, download=True)
-                    for m in request.messages
+                    await convert_message_to_openai_dict(m) for m in request.messages
                 ]
             else:
                 input_dict["prompt"] = await chat_completion_request_to_prompt(