Add get_leaderboard() public method to AutoML class

[oheliav]

Closes #806

What this PR does

Exposes get_leaderboard() as a public method on the AutoML class.

The method already existed in BaseAutoML but was not accessible
from the public API. Users call:

automl.get_leaderboard()

and get back a pandas DataFrame with all trained models and their metrics.

Changes

• Added get_leaderboard() to supervised/automl.py with full docstring
• Added test_get_leaderboard() to tests/tests_automl/test_automl.py

Testing

All existing tests pass. New test verifies the method returns a valid
DataFrame with expected columns and multiple rows.