v2: require the current state_update after session/resume #2286

In v2, sending a state_update right after session/resume is allowed but not required. A client that reconnects while a turn is running, or after it ended while disconnected, cannot tell whether foreground work is in progress unless the agent happens to send the current state. Agents that keep sessions running independently of the connection (server-side agents, several tabs on one session) need clients to learn this reliably.

Proposal: require the agent to send the current foreground state_update (with the stop reason of the last settled work when idle) as part of answering session/resume, both with and without replayFrom.