Initialize streaming with default handler that returns handle_event callback.

Options injected by the provider stream path:

• :stream_decoder — module decoding provider SSE frames (e.g. GenAI.StreamHandler.OpenAI); when nil the handler only raw-accumulates
• :stream_sink — pid or 1-arity fun forwarded each decoded event as {:genai, event} / event
• :stream_model / :stream_provider — stamped onto the final completion

Handle streaming events from Finch.stream_while Processes status, headers, data chunks, and trailers