fix(metrics): add actionable BERTScore baseline guidance

[LarryHu0217]

Fixes #995

Summary

Make a missing BERTScore rescaling baseline actionable. The error and warning now include the missing local path, the official upstream baseline URL for the selected language and model, and guidance to either save the downloaded file at that path or provide baseline_path explicitly.

This deliberately keeps baseline acquisition explicit rather than introducing an unexpected network request inside scoring.

Validation

• focused regression for the warning and error guidance;
• verified the generated official baseline URL returns HTTP 200;
• Ruff lint and format checks;
• broader metrics run reached 284 passing tests; four unrelated optional-data/hash expectation failures were recorded separately;
• git diff --check.

AI assistance disclosure: Codex was used to help prepare and validate this change.