Streams POST {baseUrl}/chat/completions over SSE (System.Net.ServerSentEvents, no NuGet) and maps delta.content, delta.reasoning_content / delta.reasoning and delta.tool_calls onto AssistantMessageEvents via AssistantMessageBuilder. Configurable through OpenAIProviderOptions so one extension serves OpenAI, OpenRouter, Ollama and other compatible servers; ships the gpt-5 / gpt-5-mini / gpt-4.1 catalogue.
