chore(proxy/roundtrip): strip upstream-request debug logging from MultiTransport

The MultiTransport's job is per-request dispatch between the embedded
NetBird transport and the stdlib transport based on the direct_upstream
context flag — about 25 lines of code. The header/body debug logging
that was bundled in pulls in:

- io.ReadAll on every request body, even when log level is above debug.
  Forces buffering of streaming POSTs (LLM completions, file uploads)
  before they reach the upstream transport.
- A header redaction list and a body-snippet cap that duplicate concerns
  already covered by netbird.go's per-roundtrip log.

netbird.go already emits method/host/url/account/duration/status/err at
debug level on every roundtrip; nothing in the private-service feature
needs the extra header+body dump.

- Drop logUpstreamRequest, formatHeaders, redactHeaderValue,
  snapshotRequestBody, and the upstreamLogBodyMax constant.
- Drop the logger field and the trailing nil arg from NewMultiTransport;
  proxy/server.go and the tests updated accordingly.

proxy/server.go

@@ -542,7 +542,7 @@ func (s *Server) initNetBirdClient() {
 // client. The split is needed so direct_upstream targets resolve DNS via the
 // proxy host's resolver instead of the tunnel's DNS.
 func (s *Server) initReverseProxy() {
-	upstreamRT := roundtrip.NewMultiTransport(s.netbird, s.Logger)
+	upstreamRT := roundtrip.NewMultiTransport(s.netbird)
 	s.proxy = proxy.NewReverseProxy(s.meter.RoundTripper(upstreamRT), s.ForwardedProto, s.TrustedProxies, s.Logger)
 }