Measure a declaration against the bus's largest message, raise a condition near it, and send the vault's export compact (issue 1689)

A declaration is one bus message and the control node's passed the limit by 2 KB. The size is checked at every send
against the connection's own limit, with a condition from 90% and a message naming size and limit when over; the
export, 138 KB compact of 165 KB indented, is sent compact.

internal/link/declare_room_integration_test.go

@@ -0,0 +1,52 @@
+//go:build integration
+
+package link
+
+import (
+	"context"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/nats-io/nats.go"
+	"github.com/novox/mesh-controller/internal/testbus"
+)
+
+// Against a real bus whose largest message is small: the limit is read from the connection, the one that fits goes
+// through, the one over is refused by us with its size and the limit, not by the client's bare error (issue 1689).
+func TestADeclarationIsMeasuredAgainstTheRealBusLimit(t *testing.T) {
+	s := testbus.StartWithMaxPayload(t, 2048)
+	conn, err := nats.Connect(s.ClientURL())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer conn.Close()
+	js, err := conn.JetStream()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := js.AddStream(&nats.StreamConfig{Name: "NODES", Subjects: []string{"mesh.node.*.declare"}}); err != nil {
+		t.Fatal(err)
+	}
+	bus := OverNATS{Conn: conn, JS: js}
+	if bus.MaxPayload() != 2048 {
+		t.Fatalf("the limit read from the connection is %d, expected 2048", bus.MaxPayload())
+	}
+	var told []int64
+	was := DeclarationMeasured
+	DeclarationMeasured = func(_ context.Context, _ string, size, limit int64) { told = append(told, size, limit) }
+	t.Cleanup(func() { DeclarationMeasured = was })
+
+	fits := []byte(`{"a":"` + strings.Repeat("x", 600) + `"}`)
+	if err := Declare(t.Context(), bus, fixedSigner{}, "novox", fits, 5*time.Second); err != nil {
+		t.Fatalf("a declaration under the limit: %v", err)
+	}
+	over := []byte(`{"a":"` + strings.Repeat("x", 1600) + `"}`)
+	err = Declare(t.Context(), bus, fixedSigner{}, "novox", over, 5*time.Second)
+	if err == nil || !strings.Contains(err.Error(), "2048 bytes") || strings.Contains(err.Error(), "maximum payload exceeded") {
+		t.Fatalf("a declaration over the limit: %v", err)
+	}
+	if len(told) != 4 || told[1] != 2048 || told[2] <= 2048 {
+		t.Fatalf("measured %v", told)
+	}
+}