Python: Fix agent_id kwarg in AzureAIAgent retrieval sample (#11057)

### Motivation and Context

The current AzureAIAgent retrieval sample uses the old `assistant_id`
kwarg to retrieve the agent. We need to fix this to use the new
`agent_id` kwarg in AzureAIAgent retrieval sample

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### Description

Fix the kwarg to use `agent_id`.

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### Contribution Checklist

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- [X] The code builds clean without any errors or warnings
- [X] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
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raises no violations
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Author: moonbox3

python/samples/getting_started_with_agents/azure_ai_agent/step7_azure_ai_agent_retrieval.py

@@ -25,11 +25,11 @@ async def main() -> None:
         DefaultAzureCredential() as creds,
         AzureAIAgent.create_client(credential=creds) as client,
     ):
-        # 1. Retrieve the agent definition based on the `assistant_id`
-        # Replace the "your-assistant-id" with the actual assistant ID
+        # 1. Retrieve the agent definition based on the `agent_id`
+        # Replace the "your-agent-id" with the actual agent ID
         # you want to use.
         agent_definition = await client.agents.get_agent(
-            assistant_id="your-assistant-id",
+            agent_id="your-agent-id",
         )
 
         # 2. Create a Semantic Kernel agent for the Azure AI agent
@@ -52,7 +52,7 @@ async def main() -> None:
         finally:
             # 6. Cleanup: Delete the thread and agent
             await client.agents.delete_thread(thread.id)
-            # Do not clean up the assistant so it can be used again
+            # Do not clean up the agent so it can be used again
 
         """
         Sample Output:

python/tests/unit/agents/azure_ai_agent/test_agent_content_generation.py

@@ -105,7 +105,7 @@ def test_generate_message_content_text_and_image():
     )
 
     thread_msg.content = [image, text]
-    step = RunStep(id="step_id", run_id="run_id", thread_id="thread_id", agent_id="assistant_id")
+    step = RunStep(id="step_id", run_id="run_id", thread_id="thread_id", agent_id="agent_id")
     out = generate_message_content("assistant", thread_msg, step)
     assert len(out.items) == 5
     assert isinstance(out.items[0], FileReferenceContent)