Add Rust unit tests for generate_predictions_seasonal

[YuminosukeSato]

Context

generate_predictions_seasonal() in sampler.rs generates post-period counterfactual predictions with seasonal state propagation. It has zero Rust unit tests.

Proposed tests

1. test_generate_predictions_seasonal_output_len — output length equals post_len
2. test_generate_predictions_seasonal_propagates_pattern — with sigma2=0, predictions reproduce known seasonal pattern
3. test_generate_predictions_seasonal_season_boundary_correct — season_duration=2 fires transitions at correct timesteps
4. test_generate_predictions_seasonal_finite_output — all predictions finite with realistic noise