Uses the real parse_args() function to get CLI defaults, so when
new arguments are added they're automatically available to all tests.
Changes:
- Add tests/conftest.py with create_args fixture
- Update 8 test files to use shared fixture
- Remove duplicate _create_mock_args methods
- Remove redundant @pytest.fixture mock_args definitions
This eliminates the need to update multiple test files when
adding new CLI arguments.
