Return axes from TFTExplainer.plot_variable_selection

[ReinerBRO]

Summary

• return matplotlib axes from TFTExplainer.plot_variable_selection() so callers can post-process plots (save/close/customize)
• keep behavior for single-series explanations simple by returning one axes array
• return a list of axes arrays when plotting multiple series
• add a focused regression assertion in test_variable_selection_explanation

Why

Issue #2892 requests access to the figure/axes from plot_variable_selection(); previously the method only showed the plot and returned None.

Validation

• . .venv/bin/activate && python -m pytest darts/tests/explainability/test_tft_explainer.py -k variable_selection_explanation -q
• . .venv/bin/activate && ruff check darts/explainability/tft_explainer.py darts/tests/explainability/test_tft_explainer.py

Fixes #2892