add GPTAssistantAgent is_termination_msg valid

[yousonnet]

Why are these changes needed?

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[sonichi]

Code formatting error can be fixed using pre-commit.

[gagb]

Thank you! This change is important because otherwise GPTAssistantAgent won't prioritize termination (and human reply) over generating its response even if is_termination_msg or human_input_mode is set.

But please fix the formatting issues.

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (d01063d) 37.79% compared to head (4ad0d5c) 49.61%.

Files	Patch %	Lines
autogen/agentchat/contrib/gpt_assistant_agent.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1642       +/-   ##
===========================================
+ Coverage   37.79%   49.61%   +11.81%     
===========================================
  Files          50       50               
  Lines        5704     5706        +2     
  Branches     1295     1404      +109     
===========================================
+ Hits         2156     2831      +675     
+ Misses       3377     2648      -729     
- Partials      171      227       +56     

Flag	Coverage Δ
unittests	49.57% <0.00%> (+11.78%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.