[bugfix] fix meta-reference agents w/ safety multiple model loading pytest

[yanxi0830]

What does this PR do?

• Fix broken pytest for meta-reference's agents
• Safety model needs to be registered to a different provider id from inference model in order to be recognized

Test Plan

torchrun $CONDA_PREFIX/bin/pytest -v -s llama_stack/providers/tests/agents/test_agents.py -m "meta_reference" --safety-shield meta-llama/Llama-Guard-3-1B --inference-model meta-llama/Llama-3.1-8B-Instruct

Before

After

Other test not broken

pytest -v -s llama_stack/providers/tests/agents/test_agents.py -m "together" --safety-shield meta-llama/Llama-Guard-3-8B --inference-model meta-llama/Llama-3.1-405B-Instruct-FP8

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.