feat(cohere): Instrument async client methods

[ericapisani]

No AsyncClient methods are wrapped. Both the OpenAI and Anthropic integrations wrap async variants (AsyncCompletions.create, AsyncMessages.create, etc.). The Cohere async client's chat, chat_stream, and embed methods should be instrumented to match.

Acceptance criteria:

• Async chat, chat_stream, and embed methods emit spans
• Behavior mirrors the sync wrappers (token usage, input/output data, error handling)
• Follows gen_ai.* attribute conventions (not deprecated ai.*)
• Sets gen_ai.system = "cohere"

[linear-code]

PY-2251