docs: update test_agents to use new Agent SDK API (#1402)

# Summary:
new Agent SDK API is added in
https://github.com/meta-llama/llama-stack-client-python/pull/178

Update docs and test to reflect this.

Closes https://github.com/meta-llama/llama-stack/issues/1365

# Test Plan:
```bash
py.test -v -s --nbval-lax ./docs/getting_started.ipynb

LLAMA_STACK_CONFIG=fireworks \
   pytest -s -v tests/integration/agents/test_agents.py \
  --safety-shield meta-llama/Llama-Guard-3-8B --text-model meta-llama/Llama-3.1-8B-Instruct
```

docs/source/building_applications/agent.md

@@ -14,18 +14,16 @@ Agents are configured using the `AgentConfig` class, which includes:
 - **Safety Shields**: Guardrails to ensure responsible AI behavior
 
 ```python
-from llama_stack_client.types.agent_create_params import AgentConfig
 from llama_stack_client.lib.agents.agent import Agent
 
-# Configure an agent
-agent_config = AgentConfig(
-    model="meta-llama/Llama-3-70b-chat",
-    instructions="You are a helpful assistant that can use tools to answer questions.",
-    toolgroups=["builtin::code_interpreter", "builtin::rag/knowledge_search"],
-)
 
 # Create the agent
-agent = Agent(llama_stack_client, agent_config)
+agent = Agent(
+    llama_stack_client,
+    model="meta-llama/Llama-3-70b-chat",
+    instructions="You are a helpful assistant that can use tools to answer questions.",
+    tools=["builtin::code_interpreter", "builtin::rag/knowledge_search"],
+)
 ```
 
 ### 2. Sessions