ipn/ipnlocal: add a C2N endpoint for fetching a netmap

For debugging purposes, add a new C2N endpoint returning the current
netmap. Optionally, coordination server can send a new "candidate" map
response, which the client will generate a separate netmap for.
Coordination server can later compare two netmaps, detecting unexpected
changes to the client state.

Updates tailscale/corp#32095

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

tstest/integration/testcontrol/testcontrol.go

@@ -336,7 +336,7 @@ func (s *Server) serveUnhandled(w http.ResponseWriter, r *http.Request) {
 func (s *Server) serveC2N(w http.ResponseWriter, r *http.Request) {
 	if err := func() error {
 		if r.Method != httpm.POST {
-			return fmt.Errorf("POST required")
+			return errors.New("POST required")
 		}
 		token, ok := strings.CutPrefix(r.URL.Path, "/c2n/")
 		if !ok {
@@ -1148,18 +1148,25 @@ func (s *Server) canGenerateAutomaticMapResponseFor(nk key.NodePublic) bool {
 func (s *Server) hasPendingRawMapMessage(nk key.NodePublic) bool {
 	s.mu.Lock()
 	defer s.mu.Unlock()
-	_, ok := s.msgToSend[nk].(*tailcfg.MapResponse)
+	_, ok := s.msgToSend[nk]
 	return ok
 }
 
 func (s *Server) takeRawMapMessage(nk key.NodePublic) (mapResJSON []byte, ok bool) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
-	mr, ok := s.msgToSend[nk].(*tailcfg.MapResponse)
+	mr, ok := s.msgToSend[nk]
 	if !ok {
 		return nil, false
 	}
 	delete(s.msgToSend, nk)
+
+	// If it's a bare PingRequest, wrap it in a MapResponse.
+	switch pr := mr.(type) {
+	case *tailcfg.PingRequest:
+		mr = &tailcfg.MapResponse{PingRequest: pr}
+	}
+
 	var err error
 	mapResJSON, err = json.Marshal(mr)
 	if err != nil {