ToUIMessageStream

Bridge an agent StreamEvent flow to the v6 UI-message-stream wire protocol: each emission is a UIMessageChunk JSON object (e.g. { "type": "text-delta", "id": "...", "delta": "..." }) ready to be framed as an SSE data: line and consumed by a JS useChat client. Events with no wire counterpart (response-metadata, tool-input-end, raw) are dropped.

This is the server-side counterpart to streamToUiMessages (which builds renderable snapshots in-process); use this when serving the stream over HTTP.