test(rest): add black-box WebSocket monitor tests (#621)

Cover the REST WebSocket monitor endpoints (v1 + v2), previously at 0%
for all 18 functions in socketv1_controller.go and socketv2_controller.go;
pkg/rest coverage goes from 20.9% to 73.2%.

The harness dials a real gorilla/websocket client into an httptest server
wired to a running msghub.Hub, and relies on the hub's history replay for
deterministic ordering: dispatching before dialing seeds the replay, and
events are relayed in dispatch order, so receiving event N next proves
filtered events were skipped rather than delayed.

Covered for both API versions:

- monitor-all and per-mailbox endpoints, including history replay and live
  delivery, and mailbox filtering of both message-stored and (v2)
  message-deleted events
- address-form mailbox names via Manager.MailboxForAddress (user%40example.com)
- invalid mailbox names rejected with 500 before the upgrade, and plain GET
  requests failing the handshake with 400
- client data messages discarded by the read pump
- clean client shutdown: listener deregistration, empty close frame from
  the writer racing the read pump's close-code echo, and the
  WebSocketConnectsCurrent expvar gauge increment/decrement

Tests only; no production changes.  Notes for follow-up commits:

- v1 monitors ignore deletions while v2 reports them (pinned as current
  behavior in TestMonitorMailboxMessagesV1)
- httptest.Server.Close does not wait for hijacked connections, so
  dialMonitor's cleanup awaits the handler goroutine via the connections
  gauge before the hub is stopped; the read/write pumps calling
  msgListener.Close concurrently can still double-close its channel
  (check-then-act race in Close's select) and deserves a fix
- deadline-set error paths, the 54s ping/pong cycle, and write-failure
  paths remain uncovered, consistent with the POP3 send-error paths

pkg/rest/testutils_test.go

@@ -51,7 +51,7 @@ func testRestPatch(url string, body string) (*httptest.ResponseRecorder, error)
 	return w, nil
 }
 
-func setupWebServer(mm message.Manager) *bytes.Buffer {
+func setupWebServer(mm message.Manager, hub *msghub.Hub) *bytes.Buffer {
 	// Capture log output
 	buf := new(bytes.Buffer)
 	log.SetOutput(buf)
@@ -63,7 +63,7 @@ func setupWebServer(mm message.Manager) *bytes.Buffer {
 		},
 	}
 	SetupRoutes(web.Router.PathPrefix("/api/").Subrouter())
-	web.NewServer(cfg, mm, &msghub.Hub{})
+	web.NewServer(cfg, mm, hub)
 
 	return buf
 }