cohere: capture rerank relevant_spans #1004

Sub-issue of #1001 (follow-ups from the 2026-10-26 dependency update, #998).

• v2 rerank gained a relevant_spans request option (Rerank v5+ models only), and each result can carry relevant_spans: [{start, end, score}].
• We drop both: the option isn't in _RERANK_METADATA_KEYS (integrations/cohere/tracing.py:81), and _rerank_output (:337) keeps only index and relevance_score.
• Fix: add the key to the metadata list and include relevant_spans per result when present. Needs one re-recorded rerank test with a v5 model.