feat: microgen - Add unit tests for list_code_objects

[chalmerlowe]

This PR introduces comprehensive unit tests for the list_code_objects() function in
scripts/microgenerator/generate.py.

The new test suite, located in tests/unit/test_generate_lister.py, covers the various functionalities
of list_code_objects(), including:

• Listing class names only.
• Showing methods, attributes, and method arguments based on boolean flags.
• Handling both single file and directory inputs.
• Testing with empty files and files containing no classes.

Mocks are used to isolate the function's logic from the file system and AST parsing