speed up notebook in CI and fix CI failures

[sonichi]

Why are these changes needed?

• Reduce time in running the notebook test in CI.
• Fix a bug for the summary method found in https://github.com/microsoft/autogen/actions/runs/8733780702/job/23963135985
• Restrict openai version due to a recent breaking change from openai, causing GptAssistant CI failure: https://github.com/microsoft/autogen/actions/runs/8737286114/job/23973931039?pr=2427

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.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 34.67%. Comparing base (4a44093) to head (dbf609a).
Report is 38 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2427      +/-   ##
==========================================
- Coverage   38.14%   34.67%   -3.47%     
==========================================
  Files          78       80       +2     
  Lines        7865     8346     +481     
  Branches     1683     1781      +98     
==========================================
- Hits         3000     2894     -106     
- Misses       4615     5206     +591     
+ Partials      250      246       -4     

Flag	Coverage Δ
unittests	34.67% <ø> (-3.46%)	⬇️

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