Use One Proxy Transport for Firebase Admin Java and Token Refresh

Pass the transport factory to GoogleCredentials and the transport to FirebaseOptions so both HTTP paths receive the proxy.

Configure both transport inputs when Firebase HTTP calls and credential refresh must use the same proxy. A public HTTP endpoint chosen from the HTTP proxy list needs this same pair of handoffs:

var proxy = new java.net.Proxy(
    java.net.Proxy.Type.HTTP,
    new java.net.InetSocketAddress("proxy.example.net", 8080));

var transport =
    new com.google.api.client.http.javanet.NetHttpTransport.Builder()
        .setProxy(proxy)
        .build();

var credentials = com.google.auth.oauth2.GoogleCredentials.fromStream(
    serviceAccountStream, () -> transport);

var options = com.google.firebase.FirebaseOptions.builder()
    .setCredentials(credentials)
    .setHttpTransport(transport)
    .build();

var app = com.google.firebase.FirebaseApp.initializeApp(options);

The two important handoffs are fromStream(serviceAccountStream, () -> transport) and setHttpTransport(transport). The first supplies the HTTP transport for obtaining access tokens; the second supplies the Firebase app's HTTP transport. Setting just one can leave the other on its default route.

serviceAccountStream is the application's trusted service-account credential stream. Keep the app for its workload and release it through the normal Firebase lifecycle. The proxy setting leaves the destination hostname intact for TLS verification; it requires neither SDK class replacement nor a trust-all SSL factory.

The scope here is the shown HTTP interfaces. A workload using Realtime Database WebSocket or another service transport needs that transport's routing support checked separately.

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