tests: fix mlflow_manager import mocking on Python 3.12

[nguyenhuulocbb]

This PR fixes an INTERNALERROR in tests/unit/test_mlflow_manager.py on Python 3.12.

• On macOS + Python 3.12, __builtins__ can be a dict, so __builtins__.__import__ raises an AttributeError.
• The test only needs to simulate ImportError for mlflow, not depend on the internal shape of __builtins__.

Changes:

• Use the builtins module and back up builtins.__import__ instead of __builtins__.__import__.
• Keep the existing behavior of raising ImportError when mlflow is imported.

After this change:

pytest tests/unit/test_mlflow_manager.py -q

Fixes #72