📁 feat: Add C# Support for Native File Search

[danny-avila]

Summary

Closes #4056

• Updated the textMimeTypes regular expression in file-config.ts to include the MIME type for C# files (x-csharp).

Note: OpenAI Assistants API does not support files ending with .cs extension. To circumvent this, you can save the code as .txt, also the planned update will help address this: #2755

Testing

Checklist

• My code adheres to this project's style guidelines
• I have performed a self-review of my own code
• My changes do not introduce new warnings
• I have tested the changes to ensure they work as expected