fix: CohereRanker reordered the top_k fallback to happen after validation

[davidsbatista]

Related Issues

• fixes https://github.com/deepset-ai/haystack-private/issues/506

Proposed Changes:

• Added init-time top_k <= 0 validation
• Reordered _prepare_cohere_input_docs (shared by run/run_async) to validate before falling back.

How did you test it?

• Fixed 3 pre-existing tests that constructed CohereRanker(top_k=-2/0) outside a pytest.raises block, those would now fail at construction time since init validates.
• Added test_run_zero_topk_does_not_fall_back_to_instance_topk and its async counterpart, the exact swallowed-fallback scenario, with a non-default instance top_k=5.

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]

Coverage report (cohere)

Click to see where and how coverage changed

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

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