CKS: Add image store validation for Kubernetes version registration

[Damans227]

Description

When registering a CKS data ISO for all zones, if any zone lacks secondary storage, the registration fails but leaves an orphaned ISO in zones that do have storage. This requires manual cleanup.

Fixes: #10994

This PR adds upfront validation to check that secondary storage exists before attempting to register the ISO:

• If a specific zone is provided, validates that zone has secondary storage
• If no zone is provided (registering for all zones), validates all zones have secondary storage and reports which zones are missing it.

Fixes: #10994

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)
• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Enhancement (improves an existing feature and functionality)
• Cleanup (Code refactoring and cleanup, that may add test cases)
• Build/CI
• Test (unit or integration test code)

Feature/Enhancement Scale or Bug Severity

Feature/Enhancement Scale

• Major
• Minor

Bug Severity

• BLOCKER
• Critical
• Major
• Minor
• Trivial

Screenshots (if appropriate):

N/A

How Has This Been Tested?

• Added unit tests for validateImageStoreForZone method.
• Manual testing against KVM based 4.20 environment.