fix: clarify structured prompt failures

crates/maestro-daemon/src/session.rs

@@ -139,6 +139,14 @@ pub enum SessionError {
     KillGuarded(maestro_protocol::InTheWay),
 }
 
+/// The result of claiming the next prompt turn for a structured ACP session.
+#[derive(Debug)]
+pub enum BeginAcpTurn {
+    Started(Arc<AcpSession>, u64),
+    InFlight,
+    Unavailable,
+}
+
 impl SessionError {
     pub fn to_error_info(&self) -> ErrorInfo {
         let code = match self {
@@ -476,25 +484,27 @@ impl SessionManager {
 
     /// Begin an ACP turn only when the previous one has ended. The projection
     /// lock makes the rejection and lifecycle update one indivisible action.
-    pub async fn begin_acp_turn(
-        &self,
-        id: Uuid,
-    ) -> Result<Option<(Arc<AcpSession>, u64)>, SessionError> {
+    pub async fn begin_acp_turn(&self, id: Uuid) -> Result<BeginAcpTurn, SessionError> {
         let (acp, stream) = {
             let sessions = self.inner.sessions.lock().await;
             let record = sessions.get(&id).ok_or(SessionError::NotFound(id))?;
+            if record.session.view != SessionView::Structured
+                || record.session.status != SessionStatus::Running
+            {
+                return Ok(BeginAcpTurn::Unavailable);
+            }
             (record.acp.clone(), record.acp_events.clone())
         };
         let (Some(acp), Some(stream)) = (acp, stream) else {
-            return Ok(None);
+            return Ok(BeginAcpTurn::Unavailable);
         };
         let mut stream = stream.lock().await;
         if stream.projection.turn_active() {
-            return Ok(None);
+            return Ok(BeginAcpTurn::InFlight);
         }
         let (turn, update) = stream.projection.begin_turn();
         let _ = stream.projected_live.send(update);
-        Ok(Some((acp, turn)))
+        Ok(BeginAcpTurn::Started(acp, turn))
     }
 
     /// Cancel an ACP turn and immediately clear its client-facing lifecycle.