fix: respect falsy run() overrides in SentenceTransformersSimilarityRanker

[keosung]

Related Issues

• fixes Falsy run() overrides are silently ignored in SentenceTransformersSimilarityRanker #3854

Proposed Changes:

run() resolved its overrides with or, which falls back on any falsy value, not just None. So run(scale_score=False) never disabled scaling, run(score_threshold=0.0) was replaced by the init value, and run(top_k=0) silently used the init top_k instead of raising the documented ValueError. This validates top_k before the fallback and switches the three fallbacks to None checks, the same change #3745 and #3746 made for the fastembed rankers. Behavior now matches the run() docstring, which already says "If set, overrides the value set at initialization."

How did you test it?

Added 3 regression tests, one per parameter. They fail on main and pass with the fix. Also verified with a real cross-encoder model that run(scale_score=False) now returns raw logits instead of sigmoid-scaled scores, and that the unit and integration suites both pass (23 + 3 tests).

Notes for the reviewer

Behavior change: run(top_k=0) now raises instead of silently using the init value, same as the fastembed rankers fix.

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test:.

[github-actions]

Heads-up for maintainers

This PR is from a fork and touches integrations whose integration tests require API keys.
Those tests are skipped in CI because fork PRs don't have access to repo secrets for security reasons.

Affected integrations:

• sentence_transformers

Please run the integration tests locally (hatch run test:integration inside each folder) before approving.

[github-actions]

Coverage report (sentence_transformers)

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
integrations/sentence_transformers/src/haystack_integrations/components/rankers/sentence_transformers
sentence_transformers_similarity.py
Project Total

This report was generated by python-coverage-comment-action

[bogdankostic]

Looks good, thanks for the PR @keosung!