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/internal/roundtrip/multi_test.go

@@ -31,7 +31,7 @@ func (s *stubRoundTripper) RoundTrip(_ *http.Request) (*http.Response, error) {
 
 func TestMultiTransport_DispatchesByContextFlag(t *testing.T) {
 	embedded := &stubRoundTripper{body: "embedded"}
-	mt := NewMultiTransport(embedded, nil)
+	mt := NewMultiTransport(embedded)
 
 	t.Run("default routes to embedded", func(t *testing.T) {
 		embedded.called = false
@@ -64,7 +64,7 @@ func TestMultiTransport_DispatchesByContextFlag(t *testing.T) {
 }
 
 func TestMultiTransport_NilEmbeddedAlwaysDirects(t *testing.T) {
-	mt := NewMultiTransport(nil, nil)
+	mt := NewMultiTransport(nil)
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
 		_, _ = io.WriteString(w, "ok")
 	}))