feat(cohere): Instrument `rerank` method

The Cohere SDK's `rerank` method (ranks documents by relevance to a query) is not instrumented. This is a significant AI operation with token costs that should be tracked.

Needs a new span operation — no existing `gen_ai.*` OP constant covers reranking.

**Acceptance criteria:**

* `rerank` calls emit spans with model, token usage, and input/output data
* Follows `gen_ai.*` attribute conventions (not deprecated `ai.*`)
* Sets `gen_ai.system = "cohere"`