Refactor MLFlow retraining logic to always use retrain method
- Removed the conditional logic for full retraining, ensuring the `retrain` method is always called.
- Updated the documentation in the `retrain_model` method to reflect the changes in retraining flow.
- Adjusted tests to verify that the `retrain` method is invoked correctly, while ensuring `train` is not called when the full retrain flag is set.
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