test: cover placement-scope behavior in non-zonal regions

[tallaxes]

Description

Adds test coverage for karpenter.azure.com/placement-scope behavior in non-zonal regions using the existing westcentralus fake environment. Follow-up to #1659.

This covers:

• Unconstrained non-zonal provisioning labels nodes with:
  • topology.kubernetes.io/zone=0
  • karpenter.azure.com/placement-scope=regional
• Explicit placement-scope=regional provisions successfully in non-zonal regions.
• Explicit placement-scope=zonal does not provision in non-zonal regions and does not attempt VM or AKS Machine creation.
• Allocation filtering rejects purely regional offerings when requirements specify placement-scope=zonal.

How was this change tested?

• make presubmit

Does this change impact docs?

• Yes, PR includes docs updates
• Yes, issue opened: #
• No

Release Note