AI wrapper baseUrl warns when pointing at PostHog AI Gateway

Patch changes

• ee6a3c8 Warn when an AI wrapper's base_url points at the PostHog AI Gateway. The gateway emits its own $ai_generation, so each call would be captured (and billed) twice. The wrapper only warns and never drops the event. Detection covers the wrapper funnels (OpenAI, Anthropic, LangChain) and the OTel span path. — Thanks @richardsolomou!