[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/agents/test_agents.py

@@ -79,18 +79,6 @@ class TestClientTool(ClientTool):
             return -1
 
 
-@pytest.fixture(scope="session")
-def text_model_id(llama_stack_client):
-    available_models = [
-        model.identifier
-        for model in llama_stack_client.models.list()
-        if model.identifier.startswith("meta-llama") and "405" not in model.identifier
-    ]
-    model_id = available_models[0]
-    print(f"Using model: {model_id}")
-    return model_id
-
-
 @pytest.fixture(scope="session")
 def agent_config(llama_stack_client, text_model_id):
     available_shields = [