Report Promptfoo runtime failures as provider errors

[sylvesterkaczmarek]

Summary

• Return unexpected runtime/API failures through Promptfoo's provider error field instead of presenting them as assistant output.
• Keep successful responses and explicit guardrail-tripwire blocks unchanged.

Motivation

The Promptfoo target currently catches any unexpected exception and returns [ERROR] ... in the normal output field. That makes infrastructure or provider failures look like model behavior to the red-team evaluator, so they can be scored as if the assistant actually produced that text.

Promptfoo's Python provider result contract has a dedicated error field for provider failures. Using it keeps evaluation failures distinct from governed assistant responses.

Validation

• successful run -> existing output behavior unchanged
• input/output guardrail tripwire -> existing [BLOCKED] output unchanged
• unexpected runtime/API exception -> empty output plus populated error

Self-review

• One-line correctness fix in one file.
• No model, policy, dependency, notebook, registry, or configuration changes.
• Searched open PRs for an existing Promptfoo provider-error fix and found none.

Maintainers may modify the branch if needed.