OpenAI wrapper now accepts per-call provider override

Minor changes

• f805e5b posthog.ai.openai.OpenAI / AsyncOpenAI now accept a per-call posthog_provider_override argument. The wrapper is commonly pointed at OpenAI-compatible endpoints (DeepSeek, Groq, Mistral, Together, Fireworks, xAI, Perplexity, Ollama, Cerebras, and various gateways) via a custom base_url, but always reported $ai_provider: "openai", which breaks PostHog's cost attribution for those calls. Passing posthog_provider_override="deepseek" (for example) sets $ai_provider on the emitted event without changing how the OpenAI-shaped response is parsed. Omitting it leaves $ai_provider as "openai", exactly as before. Covers chat completions, the Responses API, .parse(), and embeddings, across sync, async, and streaming calls. — Thanks @marco-g-pm!