net/netutil: add NewDefaultTransport to avoid http.DefaultTransport panics

Several packages built their HTTP transports with

    http.DefaultTransport.(*http.Transport).Clone()

The standard library only documents http.DefaultTransport as an
http.RoundTripper, so an application is free to replace it with a
RoundTripper that is not a *http.Transport (e.g. an instrumented or
tracing wrapper). When such an application embeds tsnet.Server, the
unchecked type assertion panics as soon as tsnet brings up its control
connection, DNS bootstrap, or log uploader.

Add netutil.NewDefaultTransport, which returns a clone of the global
when it is still the standard *http.Transport (preserving existing
behavior) and otherwise returns a fresh transport mirroring the stdlib
defaults. Route every clone site through it.

Updates #19937

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Achille Roussel <achille.roussel@gmail.com>

control/controlhttp/client.go

@@ -459,7 +459,7 @@ func (a *Dialer) tryURLUpgrade(ctx context.Context, u *url.URL, optAddr netip.Ad
 		}()
 	}
 
-	tr := http.DefaultTransport.(*http.Transport).Clone()
+	tr := netutil.NewDefaultTransport()
 	defer tr.CloseIdleConnections()
 	if optACEHost != "" {
 		// If using ACE, we don't want to use any HTTP proxy.