S1: keep the bus's "no responders" beside ErrNotAsked on an ask

A request that reached no one is asked again by mesh-sdk's busretry only when
it can see the bus's own error; the node-engine's ask dropped it, so the seat
list's first read, which fell into its holder's re-serve, was never asked
again (issue 1966).

Claude-Session: https://claude.ai/code/session_01JG73KEq4zoDfM9WtYpcQgZ

internal/link/asktool_nats_test.go

@@ -2,10 +2,12 @@ package link
 
 import (
 	"context"
+	"errors"
 	"strings"
 	"testing"
 	"time"
 
+	"git.novox.be/novox/mesh-sdk/go/busretry"
 	"github.com/nats-io/nats.go"
 )
 
@@ -37,4 +39,11 @@ func TestNatsAToolCheckIsAskedOnTheLink(t *testing.T) {
 	if err == nil || !strings.Contains(err.Error(), "nothing on this machine answers") {
 		t.Fatalf("a tool nothing serves read as %v", err)
 	}
+	// The bus's "no responders" is kept beside ErrNotAsked, so a reader that asks again while its request reached no one
+	// (busretry.Undelivered) sees it (issue 1966: the seat list's first read fell into its holder's re-serve and was never
+	// asked again).
+	_, err = (&natsLink{conn: conn, js: js, node: node}).Ask(ctx, ToolSubject("sensors", "nothing_serves_this", node), nil)
+	if !errors.Is(err, ErrNotAsked) || !errors.Is(err, nats.ErrNoResponders) || !busretry.Undelivered(err) {
+		t.Fatalf("an ask nothing serves read as %v: want ErrNotAsked and the bus's no responders, undelivered", err)
+	}
 }

internal/link/hearing_nats.go

@@ -324,8 +324,10 @@ func (l *natsLink) Ask(ctx context.Context, subject string, body []byte) ([]byte
 	case err == nil:
 		return reply, nil
 	case errors.Is(err, nats.ErrNoResponders):
-		// The server says nobody subscribes the subject: delivered to no one.
-		return nil, fmt.Errorf("%w: nothing on this machine answers %s", ErrNotAsked, subject)
+		// The server says nobody subscribes the subject: delivered to no one. The bus's own error stays beside ErrNotAsked,
+		// so a caller that asks again while a request reached no one (mesh-sdk busretry.Undelivered, as the seat list's
+		// reader does while its holder re-serves; issue 1966) sees it.
+		return nil, fmt.Errorf("%w: nothing on this machine answers %s (%w)", ErrNotAsked, subject, err)
 	}
 	if refused := l.refusal(subject, before); refused != "" {
 		return nil, fmt.Errorf("%s: %v%s", subject, err, refused)