generateStream

Streams the first available provider's real SSE token stream (all three HTTP cloud providers genuinely stream — see :llm-chat's httpCloudCapabilities).

// ponytail: no cross-provider fallback once a stream is chosen, same tradeoff // CompositeOnDeviceLlm.generateStream documents — a stream that already showed the user a // few tokens can't un-show them. Add buffered fallback if an empty/failed stream turns out to // be common enough in practice to matter.