Keep the handoff continuation message when its turn is folded away

internal/agents/manager_test.go

@@ -595,7 +595,11 @@ func TestCompactCarriesHandoff(t *testing.T) {
 			ev.AgentSessionId != nil && *ev.AgentSessionId != old.Id
 	})
 
+	continued := false
 	for _, ev := range events {
+		if ev.Kind == "user.message" && *ev.AgentSessionId != old.Id {
+			continued = ev.Payload["continuation"] == true
+		}
 		if ev.Kind == "turn.ended" && *ev.AgentSessionId == old.Id {
 			if ev.Payload["handoff"] != true || ev.Payload["inbox_item_id"] != nil {
 				t.Fatalf("handoff turn.ended %v: want handoff and no inbox summary", ev.Payload)
@@ -605,6 +609,9 @@ func TestCompactCarriesHandoff(t *testing.T) {
 			t.Fatalf("handoff prompt %v: want handoff flag", ev.Payload)
 		}
 	}
+	if !continued {
+		t.Fatalf("new session's first message lacks the continuation flag")
+	}
 
 	sessions, err := e.store.ListSessions(e.ctx, e.task.Id)
 	if err != nil {