Keep the declarations retired once their stream is gone, so nothing expects, asserts or makes NODES again (issue 1945)

A node-engine newer than the registered one, or a node asleep on an older
have, made the done test false after the start deleted NODES; the bus's
health check, probes D6 and D7 and healer H3 then expected the stream and
every node's consumer again, raised them urgent and made them anew. Release
6 sends no declaration, so a stream made again helps no node: a gone stream
is retired. The advisory watchdog also reads a consumer whose stream is gone
as gone instead of failing.

cmd/mesh-controller/composed_start_test.go

@@ -3,6 +3,7 @@ package main
 import (
 	"context"
 	"encoding/json"
+	"fmt"
 	"strings"
 	"testing"
 
@@ -110,3 +111,28 @@ func TestTheDeclarationsGoOnlyOnceEveryNodeRunsTheNewEngine(t *testing.T) {
 		t.Errorf("done with no fingerprint registered, or no node")
 	}
 }
+
+// streamsGone is a bus that holds none of the streams asked for; every other part of it is unread.
+type streamsGone struct{ jetstream.JetStream }
+
+func (streamsGone) Stream(context.Context, string) (jetstream.Stream, error) {
+	return nil, jetstream.ErrStreamNotFound
+}
+
+// Issue 1945 (the alarm of a stream the start deleted): once the declarations' stream is gone from the bus, the
+// declarations are retired whatever the nodes run now, so no raise, health check, probe or healer expects the stream
+// or a node's consumer on it again. A node-engine newer than the registered one, or a node asleep on an older have,
+// made the done test false after the stream went, and the bus's objects named it missing again (2026-10-25 14:52).
+func TestTheDeclarationsStayRetiredOnceTheirStreamIsGone(t *testing.T) {
+	retired, err := func() (retired bool, err error) {
+		defer func() {
+			if p := recover(); p != nil {
+				err = fmt.Errorf("it read what the nodes run although the stream is gone: %v", p)
+			}
+		}()
+		return declarationsRetiredOn(t.Context(), nil, streamsGone{})
+	}()
+	if err != nil || !retired {
+		t.Fatalf("with no declarations' stream on the bus: retired %v (%v), want retired", retired, err)
+	}
+}