Resume Session Stream

Resume an active stream for a session.

Called by the AI SDK's useChat(resume: true) on page load. Checks for an active (in-progress) task on the session and either replays the full SSE stream or returns 204 No Content if nothing is running.

Always replays the active turn from 0-0. The AI SDK UI-message parser keeps text/reasoning part state inside a single parser instance; resuming from a Redis cursor can skip the *-start events required by later *-delta chunks.