Python: Add check for not approved and approved for Agent Sample

[nmoeller]

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

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

Code was updated and PR has 2 approvals.