TEST/change gpt4, gpt4o serise to gpt4.1nano

[SongChiYoung]

Why are these changes needed?

Package	Test time-Origin (Sec)	Test time-Edited (Sec)
autogen-studio	1.64	1.64
autogen-core	6.03	6.17
autogen-ext	387.15	373.40
autogen-agentchat	54.20	20.67

Related issue number

Related #6361

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com./microsoft/autogen/blob/main/CONTRIBUTING.md 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.

[ekzhu]

It takes whopping 370 seconds to test autogen-ext... looks like something we need to fix.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.32%. Comparing base (b6935f9) to head (45d0470).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6375   +/-   ##
=======================================
  Coverage   78.31%   78.32%           
=======================================
  Files         217      217           
  Lines       15684    15684           
=======================================
+ Hits        12283    12284    +1     
+ Misses       3401     3400    -1     

Flag	Coverage Δ
unittests	78.32% <ø> (+<0.01%)	⬆️

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[SongChiYoung]

@ekzhu

It takes whopping 370 seconds to test autogen-ext... looks like something we need to fix.

Check each tests at autogen-ext

autogen-ext/cache_store : 0.15s
autogen-ext/code_executors : 203.89s
autogen-ext/memory : 12s
autogen-ext/models : 37s
autogen-ext/tools: 14s
autogen_ext/*.py : 146.02s

I could find out slow tests

packages/autogen-ext/tests/code_executors/test_docker_commandline_code_executor.py : 161.66s
packages/autogen-ext/tests/test_openai_assistant_agent.py : 92.66s

I will make Issue with more detaile.