feat(cohere): Instrument `rerank` method

[ericapisani]

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"

[linear-code]

PY-2248