Python: Fix agent getting started samples (microsoft#11238)

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Fix two Python agent getting started samples.

### Description

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- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
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raises no violations
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Author: TaoChenOSU

python/samples/getting_started_with_agents/azure_ai_agent/step3_azure_ai_agent_group_chat.py

@@ -85,7 +85,7 @@ async def main():
 
         try:
             # 6. Add the task as a message to the group chat
-            await chat.add_chat_message(messages=TASK)
+            await chat.add_chat_message(message=TASK)
             print(f"# {AuthorRole.USER}: '{TASK}'")
             # 7. Invoke the chat
             async for content in chat.invoke():

python/samples/getting_started_with_agents/chat_completion/step10_chat_completion_agent_structured_outputs.py

@@ -58,7 +58,7 @@ async def main():
     # 4. Invoke the agent for a response
     response = await agent.get_response(message=USER_INPUT, thread=thread)
     # 5. Validate the response and print the structured output
-    reasoned_result = Reasoning.model_validate(json.loads(response.content))
+    reasoned_result = Reasoning.model_validate(json.loads(response.message.content))
     print(f"# {response.name}:\n\n{reasoned_result.model_dump_json(indent=4)}")
 
     # 6. Cleanup: Clear the thread