test: encode image data as base64 (llamastack#1003)

# What does this PR do?

Previously, the test was failing due to a pydantic validation error
caused by passing raw binary image data instead of a valid Unicode
string. This fix encodes the image data as base64, ensuring it is a
valid string format compatible with `ImageContentItem`.

Error:

```
______________ ERROR collecting llama_stack/providers/tests/inference/test_vision_inference.py _______________
llama_stack/providers/tests/inference/test_vision_inference.py:31: in <module>
    class TestVisionModelInference:
llama_stack/providers/tests/inference/test_vision_inference.py:37: in TestVisionModelInference
    ImageContentItem(image=dict(data=PASTA_IMAGE)),
E   pydantic_core._pydantic_core.ValidationError: 1 validation error for ImageContentItem
E   image.data
E     Input should be a valid string, unable to parse raw data as a unicode string [type=string_unicode, input_value=b'\xff\xd8\xff\xe0\x00\x1...0\xe6\x9f5\xb5?\xff\xd9', input_type=bytes]
E       For further information visit
https://errors.pydantic.dev/2.10/v/string_unicode
```

Signed-off-by: Sébastien Han <[email protected]>

[//]: # (If resolving an issue, uncomment and update the line below)
[//]: # (Closes #[issue-number])

## Test Plan

Execute the following:

```
ollama run llama3.2-vision --keepalive 2m &
uv run pytest -v -s -k "ollama" --inference-model=llama3.2-vision:latest llama_stack/providers/tests/inference/test_vision_inference.py

llama_stack/providers/tests/inference/test_vision_inference.py::TestVisionModelInference::test_vision_chat_completion_non_streaming[-ollama-image0-expected_strings0] PASSED
llama_stack/providers/tests/inference/test_vision_inference.py::TestVisionModelInference::test_vision_chat_completion_non_streaming[-ollama-image1-expected_strings1] FAILED
llama_stack/providers/tests/inference/test_vision_inference.py::TestVisionModelInference::test_vision_chat_completion_streaming[-ollama] FAILED
```

The last two tests are failing because Cloudflare blocked me from
accessing
https://www.healthypawspetinsurance.com/Images/V3/DogAndPuppyInsurance/Dog_CTA_Desktop_HeroImage.jpg
but this has no impact on the current fix.


[//]: # (## Documentation)
[//]: # (- [ ] Added a Changelog entry if the change is significant)

Signed-off-by: Sébastien Han <[email protected]>

Author: leseb

llama_stack/providers/tests/inference/test_vision_inference.py

@@ -4,12 +4,12 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
+import base64
 from pathlib import Path
 
 import pytest
 
-from llama_stack.apis.common.content_types import ImageContentItem, TextContentItem, URL
-
+from llama_stack.apis.common.content_types import URL, ImageContentItem, TextContentItem
 from llama_stack.apis.inference import (
     ChatCompletionResponse,
     ChatCompletionResponseEventType,
@@ -23,7 +23,7 @@
 THIS_DIR = Path(__file__).parent
 
 with open(THIS_DIR / "pasta.jpeg", "rb") as f:
-    PASTA_IMAGE = f.read()
+    PASTA_IMAGE = base64.b64encode(f.read()).decode("utf-8")
 
 
 class TestVisionModelInference: