fix: Regionalize all container images in UseRegionalImage()

[sapphirew]

Description

UseRegionalImage() only processed Containers[0] and InitContainers[0], missing the aws-eks-nodeagent container introduced in VPC CNI v1.14.0. This caused the nodeagent image to retain the default us-west-2 ECR URI on clusters in other regions when using eksctl utils update-aws-node.

Changes:

• UseRegionalImage() now iterates over all containers and init containers
• Added isRegionalImageFormat() guard to skip already-resolved URIs
• UpdateAWSNode() sets the image format prefix for aws-eks-nodeagent by name before calling UseRegionalImage()
• Added awsNodeAgentImageFormatPrefix constant

Testing

Unit tests

• Added pkg/addons/image_test.go covering UseRegionalImage() with multiple containers, mixed format-string/resolved images, Chinese regions (.amazonaws.com.cn suffix), and empty init container slices.
• Updated pkg/addons/default/aws_node_test.go to assert Containers[1] (aws-eks-nodeagent) is regionalized in both standard (us-east-1) and Chinese (cn-northwest-1) region test cases.

Live cluster verification (eu-west-1)

1. Created a new EKS cluster (regionalize-test) in eu-west-1 with one self-managed node.
2. Deleted the managed vpc-cni addon with --preserve to convert to self-managed VPC CNI.
3. Simulated the bug by manually setting the nodeagent image to us-west-2:

   aws-node:          602401143452.dkr.ecr.eu-west-1.amazonaws.com/amazon-k8s-cni:v1.20.5-eksbuild.1
   aws-eks-nodeagent: 602401143452.dkr.ecr.us-west-2.amazonaws.com/amazon/aws-network-policy-agent:v1.2.7-eksbuild.2  ← wrong region
   

4. Ran eksctl utils update-aws-node --cluster regionalize-test --region eu-west-1 --approve with the patched binary.
5. Verified all images were correctly regionalized to eu-west-1:

   aws-node:          602401143452.dkr.ecr.eu-west-1.amazonaws.com/amazon-k8s-cni:v1.21.1
   aws-eks-nodeagent: 602401143452.dkr.ecr.eu-west-1.amazonaws.com/amazon/aws-network-policy-agent:v1.3.1  ← fixed
   aws-vpc-cni-init:  602401143452.dkr.ecr.eu-west-1.amazonaws.com/amazon-k8s-cni-init:v1.21.1
   

6. Confirmed pod was 2/2 Running — both containers pulling successfully from the correct regional ECR.

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the userdocs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes
• (Core team) Added labels for change area (e.g. area/nodegroup) and kind (e.g. kind/improvement)

BONUS POINTS checklist: complete for good vibes and maybe prizes?! 🤯

• Backfilled missing tests for code in same general area 🎉
• Refactored something and made the world a better place 🌟