support for network interface deletion on subnet

[taliandre49]

Enhance PowerVS subnet deletion logic to automatically handle attached network interfaces that block subnet removal (e.g., 409 Conflict).

Details

• Add deleteNetworkInterfaces to enumerate and delete all NICs attached to a given subnet before retrying subnet deletion.
• Introduce isNetworkInterfaceError helper to detect errors indicating subnet deletion is blocked by existing IP allocations or 409 conflicts.
• Update deletePowerSubnet to:
  - Detect NIC-related deletion failures
  - Log the condition
  - Attempt NIC cleanup
  - Return a wrapped error to trigger the existing retry mechanism

[openshift-ci]

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[openshift-ci]

Hi @taliandre49. Thanks for your PR.

I'm waiting for a openshift member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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