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/testbus/testbus.go

@@ -56,10 +56,14 @@ var buses sync.Map
 
 // Start is a server with JetStream, for this test alone: on loopback, on a port the system chooses, its
 // store in the test's own directory.
-func Start(t testing.TB) *server.Server {
+func Start(t testing.TB) *server.Server { return StartWithMaxPayload(t, 0) }
+
+// StartWithMaxPayload is Start with the largest message the server takes set to maxPayload bytes; zero keeps the
+// server's default (1 MiB, as the mesh's bus has).
+func StartWithMaxPayload(t testing.TB, maxPayload int32) *server.Server {
 	t.Helper()
 	opts := &server.Options{Host: "127.0.0.1", Port: server.RANDOM_PORT, JetStream: true, StoreDir: t.TempDir(),
-		NoLog: true, NoSigs: true}
+		NoLog: true, NoSigs: true, MaxPayload: maxPayload}
 	s, err := server.NewServer(opts)
 	if err != nil {
 		t.Fatalf("a bus for this test could not be made: %v", err)