Python: Add check for not approved and approved for Agent Sample (#12316)

### Motivation and Context

I was having a Demo with the Agents, and found that
`step06_chat_completion_agent_group_chat.py` sample always terminated
after one iteration.

The Reviewer Agent answered "not approved", which also triggered the
`Termination Strategy`, due to the fact that the word `approved` is also
in `not approved`.
To only trigger when there is a real approval I added a check if the
answer is `approved` **and** the answer also don't include `not approve`


### 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
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

---------

Co-authored-by: Chris <66376200+crickman@users.noreply.github.com>
Co-authored-by: Evan Mattson <35585003+moonbox3@users.noreply.github.com>

Author: 3 people

python/samples/getting_started_with_agents/chat_completion/step06_chat_completion_agent_group_chat.py

@@ -34,7 +34,8 @@ class ApprovalTerminationStrategy(TerminationStrategy):
 
     async def should_agent_terminate(self, agent, history):
         """Check if the agent should terminate."""
-        return "approved" in history[-1].content.lower()
+        last_message = history[-1].content.lower()
+        return "approved" in last_message and "not approved" not in last_message
 
 
 REVIEWER_NAME = "ArtDirector"