feat: resolve upstream system proxy from os settings

[GcsSloop]

Summary

• resolve upstream system proxy mode from the actual OS proxy settings
• keep environment fallback for unsupported platforms and add coverage for resolver behavior
• document the rollout plan and verify locally against the current macOS proxy on a new port

Verification

• cd backend && go test ./internal/netproxy -run 'TestResolveProxyUsesSystemProxyResolver|TestResolveProxySystemModeFallsBackToEnvironment'\n- cd backend && go test ./internal/netproxy ./internal/usagedrv/... ./internal/bootstrap\n- cd backend && go test ./internal/api ./internal/providers/...\n- local probe via system proxy 127.0.0.1:7897 with isolated routerd on 127.0.0.1:6791 returned HTTP_STATUS=200