Fix mypy errors in src/codegen/gscli/backend/typestub_utils.py

[codegen-sh]

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

1. Adding proper type hints for functions and methods that handle AST nodes
2. Using Union from typing instead of the pipe operator for better compatibility
3. Adding type safety by checking if AST nodes are of the expected type before accessing attributes
4. Adding a type variable for AST nodes to improve type checking
5. Adding explicit type casting in the _strip_internal_symbols function

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

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