Add checks for Python and uv dependencies in build process

[pooya1380m]

Why are these changes needed?

the full dotnet build requires uv and running the tests requires python because of the xlang tests.
we need to doc this somewhere.

I Add prerequisite checker with optional auto-install and setup guide link

• Introduced check-prereqs.sh and check-prereqs.ps1 to validate presence of required tools (python3, uv)
• Added optional '--install-missing' flag to install missing tools automatically

Related issue number

#5734

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.

[pooya1380m]

Hi @ekzhu @lokitoth , I noticed this PR hasn’t been merged yet, and I wanted to check if there’s anything blocking it. I’m more than happy to make adjustments, add tests, or align it better with the project’s direction. Please let me know what you think. Thanks for your time and for maintaining this project!

[ekzhu]

Can we simply document this without introducing more scripts.

cc @lokitoth