feat(cohere): Instrument `classify` method

The Cohere SDK's `classify` method (text classification using examples) is not instrumented. This is an AI inference call that should emit spans with model, token usage, and input/output data.

**Acceptance criteria:**

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