fix: add status field to service list/detail APIs, unify observeStats retry pattern, fix router bugs

- api: add fillServiceStatus helper and call it from getServiceList/getService
  so status field appears in service list and detail API responses
- observeStats: unify retry pattern across all 9 handlers (http, http2, masque,
  relay, router, socks4, socks5, tungo, tunnel) — buffer events on failure,
  continue to skip fresh collection, clear on success; fix event-loss bug
  where interim events were dropped during retry cycles
- handler/router: check WriteTo/Write return errors in associate.go and
  entrypoint.go; fix DelConnector and ConnectorPool.Del using RLock instead
  of Lock (write under read lock); remove unused fields t and cancel

handler/router/entrypoint.go

@@ -48,7 +48,9 @@ func (h *routerHandler) handleEntrypoint(log logger.Logger) error {
 
 			if c := h.pool.Get(routerID, gateway); c != nil {
 				if w := c.Writer(); w != nil {
-					w.Write(buf[nn:])
+					if _, werr := w.Write(buf[nn:]); werr != nil {
+						log.Error(werr)
+					}
 				}
 			}