TEST: skip when macos+uv and adding uv venv tests

[SongChiYoung]

Why are these changes needed?

The pytest tests test_local_executor_with_custom_venv and test_local_executor_with_custom_venv_in_local_relative_path located in packages/autogen-ext/tests/code_executors/test_commandline_code_executor.py fail when run on macOS (aarch64) using a Python interpreter managed by uv (following the project's recommended development setup).

The failure occurs during the creation of a nested virtual environment using Python's standard venv.EnvBuilder. Specifically, the attempt to run ensurepip inside the newly created venv fails immediately with a SIGABRT signal. The root cause appears to be a dynamic library loading error (dyld error) where the Python executable inside the newly created venv cannot find its required libpythonX.Y.dylib shared library.

So, when MacOS + uv case, skipping that test.
And, adding uv-venv case

Related issue number

Closes #6341

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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.36%. Comparing base (cbd8745) to head (83f0caf).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6387   +/-   ##
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  Coverage   78.36%   78.36%           
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  Files         221      221           
  Lines       16020    16020           
=======================================
  Hits        12554    12554           
  Misses       3466     3466           

Flag	Coverage Δ
unittests	78.36% <ø> (ø)

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