[Beginner]: Add docstring to `TestCategorizeJob.test_returns_required_keys` method

Description

The unit test test_returns_required_keys in tests/test_categorize.py lacks an explanatory docstring. Add one explaining what it asserts.

Why it matters

Tests serve as living documentation. Clear docstrings help new contributors understand what a test's objective is.

Acceptance Criteria

• [ ] Add a clear docstring to the specific test method
• [ ] Passes pre-commit run --all-files