Resume a session's stream.

With turn, serves exactly that turn's stored entries after after (0-0 for all of them) and then its live ones: 409 when entries after the cursor were trimmed, 410 when the stream is gone. Every entry frame carries id: <turn>:<entry>.

Without turn, replays the active turn from 0-0, or returns 204 when nothing is running: the AI SDK's parser cannot continue a block it did not see start.