feat(cohere): Instrument `ClientV2` methods

[ericapisani]

The Cohere V2 client (cohere.ClientV2) exists but its methods are not patched. The current integration only patches BaseCohere and Client. If ClientV2 doesn't inherit from BaseCohere, calls through the V2 client go completely unmonitored.

Need to verify the inheritance chain and patch accordingly.

Acceptance criteria:

• ClientV2.chat, embed, and other relevant methods emit spans
• Follows gen_ai.* attribute conventions (not deprecated ai.*)
• Sets gen_ai.system = "cohere"

[linear-code]

PY-2250