Validate LLM judge concurrency

[sylvesterkaczmarek]

Summary

• Reject non-positive concurrency values at the judge_folder() boundary.
• Keep all valid worker counts and judging behavior unchanged.

Motivation

judge_folder() accepts concurrency directly and later passes it to ThreadPoolExecutor(max_workers=concurrency). Values such as 0 or -1 are therefore accepted by the function and CLI, perform some setup work, and then fail with an executor-internal ValueError.

This parameter is part of the evaluator's public helper/CLI surface, so invalid worker counts should fail immediately with a clear, local validation error before creating output directories, loading prompts, or constructing the client.

Validation

The new guard enforces the executor's existing requirement explicitly:

• concurrency < 1 -> ValueError("concurrency must be at least 1")
• concurrency >= 1 -> existing behavior unchanged

No API requests, judge prompts, retry behavior, or result formats are changed.

Self-review

• Three added lines in one file.
• Valid concurrency values preserve existing behavior.
• No dependencies, notebooks, registry entries, or documentation changed.
• Searched open PRs for an existing concurrency-validation fix and found none.

Maintainers may modify the branch if needed.