fix: add utf-8 encoding to open() calls to prevent UnicodeEncodeError…

[AimelHassan]

… on Windows

Why are these changes needed?

Fixes #5566

This PR adds `encoding="utf-8"` to file `open()` calls to prevent `UnicodeEncodeError` crashes on Windows default environments (which use      

cp1252) when encountering emojis or special characters.

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.