fix: enhance ARM shape detection and add nil check in setNodepoolImageId

[alcampag]

• Replace simple string check for ARM shapes with regex pattern Standard\.A\d+ for more accurate detection of A-series (ARM) shapes
• Initialize NodeSourceViaImage if nil to prevent potential nil pointer dereference errors
• Update test cases to use realistic ARM shape name "VM.Standard.A1.Flex" instead of placeholder
• This improves reliability and accuracy in OCI managed machine pool node pool creation

Here the template used to verify the nil pointer exception cluster-test-native.yml

Checklist

• All unit tests are passing
• All PRBlocking tests are passing
• Code builds successfully