Fix mypy errors in src/codegen/gscli/generate/system_prompt.py

[codegen-sh]

This PR fixes mypy errors in the src/codegen/gscli/generate/system_prompt.py file by:

1. Adding proper type hints to functions and variables:

   • Added return type annotations to functions
   • Added type annotation for the mint variable
   • Added parameter type annotation for the name parameter in get_group
   • Used capitalized type names from the typing module (List, Dict, etc.)

2. Improving error handling:

   • Added explicit return None in the get_group function
   • Added filtering of None values in the render_groups function

3. Adding docstrings to functions that were missing them:

   • Added docstrings to render_page, render_group, get_group, and render_groups

These changes improve type safety and code clarity without changing the functionality of the code.

Note: This is a small PR focused only on fixing mypy errors in the system_prompt.py file as requested.