fix: extend zero_division parameter to percentage and range-based metrics

[mahimn01]

Checklist before merging this PR:

• Mentioned all issues that this PR fixes or addresses.
• Summarized the updates of this PR under Summary.
• Added an entry under Unreleased in the Changelog.

Mirrors the design of #3059 for the percentage / range-based metric family. Adopts the breaking-change 🔴 marker convention discussed in #3080.

Summary

Percentage and range-based metrics (wmape, ope, arre, marre, coefficient_of_variation) previously either raised a hard ValueError or silently returned nan/inf when their denominator was zero. This made batch evaluation pipelines brittle for constant or all-zero components.

This PR extends the zero_division parameter pattern introduced in #3059 to these five metrics:

• "warn" (default) — returns np.nan and emits a warning.
• "raise" — preserves the legacy ValueError.

A new _safe_pct_divide helper sits next to _safe_scaled_divide in darts/metrics/utils.py. The two differ only in fill semantics: percentage metrics multiply the ratio by 100, so a 1.0 fill for the 0/0 case (the scaled-metric "on par with naive") would surface as 100 % error and be misleading — hence np.nan instead.

Two adjacent bugs are fixed in the same change:

• ope previously checked sum > 0 and rejected actual_series with a strictly negative sum (e.g. financial return series). The check is now sum != 0 via the helper.
• wmape's docstring claimed ValueError if actual_series contains some zeros, but the implementation divides by sum(|y_true|) and only the all-zero case ever triggered any path. Docstring corrected.

Behavior change

Input shape	Behavior
Healthy inputs (non-zero denominator)	Unchanged — all 455 existing tests pass without modification
Zero denominator + default "warn"	Returns np.nan and logs a warning (previously: silent NaN/inf, or hard ValueError)
Zero denominator + "raise"	Raises ValueError (legacy behavior preserved as opt-in)
Negative-sum actual_series in ope	Now works correctly (previously: incorrectly raised)

Per the discussion in #3080, the CHANGELOG entry for the parameter addition carries the breaking-change 🔴 marker because the default behavior flips from raising to warning.

Test plan

• New parametrized test_pct_metrics_zero_division covering all 5 metrics: warn-mode returns NaN with warning, raise-mode raises ValueError, invalid string rejected, healthy inputs unaffected.
• New test_ope_accepts_negative_sum verifying the sum != 0 fix using a series with a strictly negative sum.
• Updated existing test_ope_zero and test_arre to opt into legacy behavior with zero_division="raise".
• uv run pytest darts/tests/metrics/test_metrics.py → 461 passed (was 455, +6 new).
• uv run pre-commit run --files <changed> → all hooks pass (ruff, ruff-format, trailing whitespace, end-of-file).

Other Information

Files changed (+208/-31):

• darts/metrics/utils.py — _safe_pct_divide helper
• darts/metrics/metrics.py — 5 metrics extended
• darts/tests/metrics/test_metrics.py — 6 new tests + 2 legacy-test updates
• CHANGELOG.md — entries under **Fixed** (the bug fixes) and **Improved** (the new parameter, with 🔴 marker)