Validate ConformalIntervals horizon

[DresdenGman]

Summary

• reject non-positive ConformalIntervals horizons at construction time
• add regression coverage for zero and negative horizons

Why

A non-positive horizon currently reaches _conformity_scores, where h=0 raises a bare ZeroDivisionError. Failing at configuration time makes the invalid input clear and avoids an unrelated-looking downstream error.

Tests

• uv run pytest tests/test_models.py --no-cov -q — 206 passed
• uv run ruff check python/statsforecast/utils.py tests/test_models.py

Fixes #1221

[codspeed-hq]

Merging this PR will not alter performance

✅ 38 untouched benchmarks

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_{Comparing DresdenGman:fix/conformal-interval-h-validation (95d894b) with main (8b28576)}