[bugfix] fix broken vision inference, change serialization for bytes

[yanxi0830]

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

After

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,
  Pull Request section?
• Updated relevant documentation.
• Wrote necessary unit or integration tests.