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/relay/observe.go

@@ -33,17 +33,21 @@ func (h *relayHandler) observeStats(ctx context.Context) {
 	for {
 		select {
 		case <-ticker.C:
+			// Try to flush any buffered events from a previous failed attempt.
 			if len(events) > 0 {
-				if err := h.options.Observer.Observe(ctx, events); err == nil {
-					events = nil
+				if err := h.options.Observer.Observe(ctx, events); err != nil {
+					continue
 				}
-				// fallthrough: still collect fresh events for this tick
 			}
 
-			evs := h.stats.Events()
-			if err := h.options.Observer.Observe(ctx, evs); err != nil {
-				events = evs
+			// Collect and send fresh events.
+			if evs := h.stats.Events(); len(evs) > 0 {
+				if err := h.options.Observer.Observe(ctx, evs); err != nil {
+					events = evs
+					continue
+				}
 			}
+			events = nil
 
 		case <-ctx.Done():
 			return