Add retry logic for ExtPolicy "ResourceNotFound" test failure

[mgunnala]

Description

Issue #

We are seeing intermittent test failures for the ExtPolicy (ext_policy.py) scenario on Mooncake and Government clouds. The failures are only seen in the following scenario (test case 4):

• block custom script with policy, then try to delete -> fails as expected
• allow custom script with policy and retry the delete -> succeeds as expected
• enable custom script -> this should succeed, but we occasionally see a "ResourceNotFound" error

2025-01-28T13:41:18Z.314 [ERROR] ******** [Failed] ExtPolicy: Fail: Unexpected error while trying to enable Microsoft.Azure.Extensions.CustomScript. Extension is allowed by policy so this operation should have completed successfully.
Error: (ResourceNotFound) The Resource 'Microsoft.Compute/virtualMachines/lisa-WALinuxAgent-20250128-130736-786-e72-n0/extensions/CustomScript' under resource group 'lisa-WALinuxAgent-20250128-130736-786-e72' was not found. For more details please go to https://aka.ms/ARMResourceNotFoundFix
Code: ResourceNotFound
Message: The Resource 'Microsoft.Compute/virtualMachines/lisa-WALinuxAgent-20250128-130736-786-e72-n0/extensions/CustomScript' under resource group 'lisa-WALinuxAgent-20250128-130736-786-e72' was not found. For more details please go to https://aka.ms/ARMResourceNotFoundFix!

As a workaround, this PR adds logic to retry 'enable' if the operation fails with a ResourceNotFound error.

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