fix: 捕获钉钉 Stream SDK 内部 panic，避免崩溃

钉钉 SDK v0.9.1 在连接断开时有 send on closed channel panic，
加 defer recover 捕获后自动重连。

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

platform/dingtalk/dingtalk.go

@@ -181,9 +181,16 @@ func (p *Platform) Start(handler core.MessageHandler) error {
 			default:
 			}
 
+			func() {
+				defer func() {
+					if r := recover(); r != nil {
+						slog.Error("dingtalk: stream panic, reconnecting", "recover", r)
+					}
+				}()
 				if err := p.streamClient.Start(ctx); err != nil {
 					slog.Warn("dingtalk: stream disconnected, reconnecting", "error", err)
 				}
+			}()
 
 			// Brief pause before reconnecting to avoid tight loop on persistent failures.
 			select {