fix: wait for server tailnet background routines to exit on Close (#15183)

fixes https://github.com/coder/internal/issues/114

We need to wait for ServerTailnet goroutines to finish when closing down, otherwise we can race with the shutdown of coderd & the coordinator, which causes errors.

enterprise/wsproxy/wsproxy.go

@@ -419,6 +419,8 @@ func (s *Server) RegisterNow() error {
 }
 
 func (s *Server) Close() error {
+	s.Logger.Info(s.ctx, "closing workspace proxy server")
+	defer s.Logger.Debug(s.ctx, "finished closing workspace proxy server")
 	s.cancel()
 
 	var err error