Detect and fix rare cases where the primary ENI does not serve default traffic

During testing, we encountered a rare scenario when launching EC2 instances with multiple ENIs: the primary ENI (device index 0) does not serve default network traffic. This occurs in approximately 1 out of 10,000 launches (0.01%). For example, when configuring two ENIs on an instance—ENI-0 (deviceIndex=0) from subnet-0 and ENI-1 (deviceIndex=1) from subnet-1—Linux may recognize eth0 as being from…