[client sdk test] add options for inference_model, safety_shield, embedding_model (#843)

# What does this PR do?
Default inference_model for testing: "meta-llama/Llama-3.1-8B-Instruct"
Default vision inference_model for testing:
"meta-llama/Llama-3.2-11B-Vision-Instruct"


## Test Plan
`/opt/miniconda3/envs/stack/bin/pytest -s -v
--inference-model=meta-llama/Llama-3.2-3B-Instruct
tests/client-sdk/agents`


`/opt/miniconda3/envs/stack/bin/pytest -s -v
--embedding-model=all-MiniLM-L6-v2 tests/client-sdk/vector_io`

`/opt/miniconda3/envs/stack/bin/pytest -s -v
--safety-shield=meta-llama/Llama-Guard-3-1B tests/client-sdk/safety`

## 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.

tests/client-sdk/safety/test_safety.py

@@ -32,16 +32,6 @@ def available_shields(llama_stack_client):
     return [shield.identifier for shield in llama_stack_client.shields.list()]
 
 
-@pytest.fixture(scope="session")
-def llama_guard_text_shield_id(available_shields):
-    if "meta-llama/Llama-Guard-3-1B" in available_shields:
-        return "meta-llama/Llama-Guard-3-1B"
-    elif "meta-llama/Llama-Guard-3-8B" in available_shields:
-        return "meta-llama/Llama-Guard-3-8B"
-    else:
-        pytest.skip("Llama-Guard shield is not available. Skipping.")
-
-
 @pytest.fixture(scope="session")
 def code_scanner_shield_id(available_shields):
     if "CodeScanner" in available_shields: