chore: Add powershell path check for code executor

[lspinheiro]

Why are these changes needed?

Addresses the follow up error in #5518 , it will now check if both powershell or pwsh are in the path and throw a value error if neither is available.

Related issue number

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

Attention: Patch coverage is 16.66667% with 5 lines in your changes missing coverage. Please review.

Project coverage is 77.25%. Comparing base (b1ae4ac) to head (a06cb95).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...ogen-ext/src/autogen_ext/code_executors/_common.py	16.66%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6212      +/-   ##
==========================================
- Coverage   77.27%   77.25%   -0.03%     
==========================================
  Files         200      200              
  Lines       14238    14243       +5     
==========================================
+ Hits        11003    11004       +1     
- Misses       3235     3239       +4     

Flag	Coverage Δ
unittests	77.25% <16.66%> (-0.03%)	⬇️

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