[seanmcrae]

Summary

Handle the degenerate bpref cases using the TREC eval convention: when a query has no judged nonrelevant documents, each retrieved relevant document contributes 1; when it has no relevant documents, its score is 0. This prevents NaN scores for positive-only qrels. Regression coverage includes partial relevant retrieval (expected score 1/2), unjudged retrieved documents, and a custom rel_lvl.

Validation

• Against base 7363db0c35e92e90d6fa6fe73907b760678f765e, both partial-retrieval parameter cases returned nan and failed the expected 0.5 assertion.
• /tmp/ranx-test-venv/bin/pytest -q tests/unit/ranx/metrics: 10 passed.
• isort, Black, and typos checks pass on changed files. Ruff passes on the test file; it flags two pre-existing UP007 findings on unchanged Union[...] signatures in ranx/metrics/bpref.py (lines 52–53).