text: Title: [BUG] ShapExplainer summary_plot Horizon does not include output_chunk_shift
  Body: **Describe the bug**
  `ShapExplainer().summary_plot()` should include `output_chunk_shift` for the horizon in the plot.
  
  **To Reproduce**
  ```python
  from darts.datasets import AirPassengersDataset
  from darts.explainability import ShapExplainer
  from darts.models import LinearRegressionModel
  
  series = AirPassengersDataset().load()
  model = LinearRegressionModel(lags=4, output_chunk_shift=4)
  model.fit(series)
  shap_explainer = ShapExplainer(model=model)
  shap_values = shap_explainer.summary_plot()
  ```
  gives 
  
  ![Image](https://github.com/user-attachments/assets/db29e4a4-da23-40ce-b91d-3f83f05ac9af)
  
  **Expected behavior**
  The plot title should be `Horizon: t+(1+output_chunk_shift)` i.e. `Horizon: t+5`. 
  
  **System (please complete the following information):**
   - Python version: 3.12.7
   - darts version: 0.32.0
  