Add PATHEXT to default STDIO env vars in windows

[timesler]

Add PATHEXT to default env vars in windows

Motivation and Context

The PATHEXT env var is required for the subprocess to be able to find many executable and other files

How Has This Been Tested?

Tested before and after with a STDIO server that provides powershell tools. Only works with this change in place

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

[ochafik]

Hi @timesler, thanks for sending this!

Looks straightforward / goes hand in hand w/ PATH on windows.