[bugfix] fix broken vision inference, change serialization for bytes (#693)

# What does this PR do?

- vision inference via image as binary bytes fails with serialization
error
- add custom serialization for "bytes" in `_URLOrData`

## Test Plan

```
pytest -v -s -k "fireworks" --inference-model="meta-llama/Llama-3.2-11B-Vision-Instruct" ./llama_stack/providers/tests/inference/test_vision_inference.py::TestVisionModelInference::test_vision_chat_completion_non_streaming
```

**Before**
<img width="1020" alt="image"
src="https://github.com/user-attachments/assets/3803fcee-32ee-4b8e-ba46-47848e1a6247"
/>


**After**
<img width="1018" alt="image"
src="https://github.com/user-attachments/assets/f3e3156e-88ce-40fd-ad1b-44b87f376e03"
/>

<img width="822" alt="image"
src="https://github.com/user-attachments/assets/1898696f-95c0-4694-8a47-8f51c7de0e86"
/>



## 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/apis/common/content_types.py

@@ -4,11 +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 typing import Annotated, List, Literal, Optional, Union
 
 from llama_models.schema_utils import json_schema_type, register_schema
 
-from pydantic import BaseModel, Field, model_validator
+from pydantic import BaseModel, Field, field_serializer, model_validator
 
 
 @json_schema_type
@@ -27,6 +28,12 @@ class _URLOrData(BaseModel):
             return values
         return {"url": values}
 
+    @field_serializer("data")
+    def serialize_data(self, data: Optional[bytes], _info):
+        if data is None:
+            return None
+        return base64.b64encode(data).decode("utf-8")
+
 
 @json_schema_type
 class ImageContentItem(_URLOrData):